It is pretty rare for this to happen, but I really disagreed with certain elements of my brother's last post concerning transformation abilities in various videogames. For various reasons, I like many of the transformations systems that he didn't like and don't like a lot of the systems that he did.
For example, I love Shadow Hearts' demon transformation system. It is a system that lets the main hero, Yuri, be both the most flexible character in the game and still prevents him from being able to do everything at once. If you transform into a powerful Light form like Sandalphon, then you gain access to strong light-elemental attack and get vastly improved magical defense, and if you transform into a Fire form like Forron you can use fire spells and get greatly improved physical strength. In his base form, Yuri can only either use a basic attack or transform, so these specialized demon forms and their unique powers make up the entirety of his ability set. They cost Sanity Points, which are fully restored at the end of each battle, but because it is actually much harder to restore Sanity Points during a battle than something like HP or Magic Points, and powerful transformation cost very large amounts of Sanity, it means that you can't freely switch forms in the midst of a fight. Once you have transformed, your SP will be too low to risk another transformation. Thus, the real cost of each form is the opportunity cost of the abilities granted by every other form, which works very well since each individual form has an incredibly limited array of moves. Rather than being an ability too costly to use often, Yuri's demon transformation is the core of his power.
As for the entire "dramatic tension for boss battles" argument, I will pretty much completely disagree. Just because Yuri can use his strongest transformations in every battle doesn't mean that his transformations can't add a lot of dramatic tension to a boss battle. For one thing, my brother forgot to mention the one use for Sanity other than transforming: warding off the "Berserk" state. Every turn each character's Sanity decreases slightly, and when it runs out that character loses it and goes "Berserk". This really doesn't come into play for any character during normal battles, but it is a major factor in every boss battle. Depending on various factors, Yuri's demon transformation ability can make him extremely susceptible to this (using the penultimate Amon form frequently reduced Yuri to merely 3 Sanity when it was new, even with a cost-reducing accessory equipped). The boss fights are difficult enough that Yuri needs to use his strongest (and most costly) forms in order to win, so even though he has far more Sanity than almost anyone else, his transformations level the playing field so he is in the same tense situation as everyone else. Also, the fact that Yuri needs to use his strongest forms and powers magnifies a slightly hidden cost to using powerful transformations: the cost of the powers granted to each form.
Yuri's third-tier Dark form, Czernoborg, is much stronger than his first-tier Dark form, Death Emperor. Also, Czernoborg's main attack spell, Revelation, is much stronger than Death Emperor's Dark Messenger spell. However, just as the Czernoborg transformation costs nearly four times as much Sanity as the Death Emperor transformation, the Revelation spell costs nearly four times as much MP as the Dark Messenger spell. In addition, Revelation is a much more difficult spell to successfully cast, thanks to the game's Judgement Ring system. Revelation is the kind of spell you need to use in many of the game's later battles, but because Mp doesn't get restored between battles, it is just too costly and tricky to use often against normal enemies. If you want to actually use attack spells in normal battles, a weaker form with cheaper spells is a better strategy. Also, the powerful stat-boosting spells that define the different elements are only really useful in boss battles. As such, there was a pretty clear difference in both the forms I used and the way I used transformations between normal battles and boss battles.
Finally, I will say that the fact that you can use Yuri's demon transformation in every battle is a valuable benefit in of itself. If a character has a particularly defining ability, especially one that requires a lot of effort to manage and develop, then that ability should be useable as often as possible. If you work hard to unlock an ability, then you should be able to use that ability more than once. Since unlocking new demon forms for Yuri can be time consuming and tricky, that effort should be rewarded. If you could only justify using such forms in boss battles, then you get the situation where you have only four chances to use one of six different forms, which means a lot of effort can go to waste.
For many reasons related to why I like the Shadow Hearts demon transformation system, I don't really like the Breath of Fire 3 and 4 dragon transformation system. Where Shadow Hearts elegantly separates the cost of transforming and the cost of using a form's abilities so they use different resources, in the Breath of Fire games they both share the same resource, often leading to the transformation ending too quickly and many abilities being too costly (both transformation cost and ability cost increasing with more powerful forms is practically double-penalizing the player in such a situation). Also, particularly with Breath of Fire 3, the Dragon Gene system is a fairly large sub-system of the game that can result in countless forms, but you never really have a chance to use that many across the course of the game (the poor balance between the forms adds to this issue, since only a limited few forms are any good). As a whole, it results in a game where you simply can't use your main character's most distinctive abilities (or hardly any abilities at all), except for just a few rounds out of the occasional boss fight, and even then you will usually only be launching cheap, ordinary attacks.
As for some of the other things my brother mentioned, I agree that Knight Blazer is a good transformation (it is not central to the character's combat ability, so the fact that you can only really use it rarely works well, and the music change is a nice touch), and that Devil May Cry's Devil Trigger is nearly perfect. I just don't agree that Metis's Neo Orgia Mode is bad, though. I think my brother underestimates the real opportunity cost of the way Metis's AI routines change while in that mode, but I do agree that the fact you can't use it until the end of a boss battle is a bit problematic (it might be better if there was a finite limit to how often it could be used in a given battle, rather than the "Metis becomes helpless afterwards" limitation). I think that about covers it...