Status effects like poisoned, blinded, asleep, and so-on have been a mainstay of the console RPG genre since it's earliest beginnings, and they can even trace their roots back to the Pen-and-Paper RPGs the genre grew out of. Yet, status effect spells have a history of being neglected and useless. In most RPGs that I have played, the opportunity cost of using a status effect spell is far too great compared to the benefit of the spell in order to make using the spell worth it.
The fundamental problem of status effect spells in console RPGs is that the most RPGs are built on the assumption that the player will fight a large number of battles, each of which takes a relatively short amount of time. However, status effects are generally designed to weaken and inconvenience enemies, not kill them quickly. For example, a status condition that gives an enemy a 50% chance of losing its turn is only useful if the enemy lives long enough to take two, three, or more turns. As a result, it is not worth using a status effect on most enemies, since they can easily be killed by one or two good attacks in most RPGs. That leaves only only two situations where status effects are useful: in unusually long regular battles, such as against a large group of enemies or particularly durable regular enemy, and boss battles. Unfortunately, this is where most RPGs drop the ball, since flat-out immunity to status effects are insanely common in RPGs. The average boss is immune or incredibly resistant to every status condition. While this is somewhat reasonable, considering that many status effects render an enemy completely powerless, the fact that even regular enemies are often given complete immunity to a wide range of status conditions, often for no discernible reason. Furthermore, status effect spells often are given a very low chance of taking effect. Because of this, using a status effect spell is a huge gamble, and is thus a pretty poor choice compared to just doing reliable damage.
I don't think there is any good justification to making status effect spells useless. If you are going to give the player an option, that option should at the very least be situationally useful. Thankfully, there are games out there where status effects are useful or even invaluable. Here are some of my observations and my advice to anyone designing status effect attacks for an RPG:
-Give the player information about the enemies' immunity to status effects.
Simply being uncertain about whether or not an enemy is susceptible to a status condition is often enough to prevent the player from using a status spell. This is why it is important to give the player at least some feedback about an enemy's resistances and vulnerabilities. For example, in Final Fantasy X it is possible to use the Scan spell to see what status effects an enemy is immune to. Furthermore, FFX would give an "Immune" message if you hit it with a status effect it was immune to, instead of "Miss". That is a very important distinction, particularly if status effect inducing attacks have a low chance of success. Without such an indicator, the player may not realize an enemy is immune to an effect and try using that effect over and over on that enemy, or just write off an enemy as being immune after a single miss. Giving the player at least some information and feedback gives the player the ability to make informed decisions.
-Status effects with a low activation rate have no value.
At one point, I considered pairing a status effect to a physical attack to be a good idea, since it meant that the attack would still do some damage, even if the status effect itself didn't take hold. However, I have since realized that there are some caveats on this principle. Let's say there is an attack called Paralyzing Strike which costs 2 MP and does damage equal to the player's basic attack with an added 20% chance of paralyzing an enemy. While it sounds like a good deal at first, the reality is the player can't afford to use this attack in lieu of a basic physical attack. Even at a low MP cost, using Paralyzing Strike every turn can quickly add up to hundreds of MP over the course of a dungeon. As a result, the player is likely to save Paralyzing Strike for use against specific targets that he specifically wants to paralyze. However, if the paralysis effect is not reliable, the player will grow frustrated with Paralyzing strike, and will switch to using more reliable strategies, such as using high MP cost, high damage attacks. As a result of this effect, status effect inducing attacks are only useful if they are either reliable or free, such as in the case of weapons that have some chance of causing a status effect with every hit (though even those generally have to compete with weapons with different effects).
-Useful status effect attacks come in two general categories:
a) Status effects that shut down the enemy's ability to do damage.
In Dragon Quest IV, I have run into enemy groups consisting of four or more enemies that both capable of dealing a lot of damage and tough enough to require the concentrated attack power of multiple characters to bring down. I have also discovered that the Snooze spell is very helpful against these enemies, since it is cheap, hits an entire group of enemies, and can shut down their attacks for several turns. When a status effect spell can swing an entire battle from very dangerous to well-under control, it becomes a very attractive option.
A similar example is with Iron Giants and Blind in Final Fantasy X-2. Even though Iron Giants are powerful attackers that can nearly flatten a character in one hit and can withstand several rounds worth of punishment, they are susceptible to the blind condition, which makes them miss almost every attack. So having a character use a blind-inducing attack on an enemy every turn is a sound strategy.
b) Status effects that allow the player to defeat enemies faster.
This should be where the poison status condition goes, but it is rarely useful, despite being in every RPG since the original Dragon Quest. In most cases, it does about 10% of an enemy's max HP in damage every turn, which is only useful if the player can only hit that enemy for a comparable amount of damage with each attack. So it is useless against most enemies, which will likely die in a turn or less of concentrated attacks, but it would be wildly overpowered against a boss which is expected to last a dozen or more rounds (thus, most bosses are immune to poison). The only time I have ever used poison as a serious combat strategy was in Final Fantasy X, where poison did one fourth of an character's max HP in damage every turn. That was powerful enough for me to use it as my primary means of killing certain enemies (such as the apes on Mt. Gagazet).
Perhaps a more interesting version of this kind of status condition are effects that increase the damage the player will do against enemies. For example, the oil condition in Final Fantasy XII caused the next Fire elemental attack to do double damage, which made it into one of my main boss-killing strategies. Better still is Persona 3's Distress condition, which makes every attack against the Distressed target into a critical hit. Because of the way criticals work in Persona 3, a spell that makes every enemy distressed can let the player mow down an entire enemy party with just basic attacks (if the condition actually connects, that is).
Status spells can be very useful, and open up more complex and interesting strategies to the player, if the developers of an RPG let them. However, unreliability is the bane of usefulness; there is no reason to give the player a skill that does not do its job. If you design status effects with particular uses in mind, then they will be useful. Far too often, it seems like status conditions are designed the monsters to use against the PCs, but then are given to the player with little though put into their usefulness.