My brother and I usually play through RPGs together. While only one of us actually plays through the game and makes the final decisions, the other watches through the game to enjoy the story and help remember details, think through puzzles, and generally try to make helpful decisions. The ability to allow my brother to help out in a more direct fashion while I played through a game was something I enjoyed. It did not make combat any easier, since only one of us could make decisions at a time. However, it was a very different experience, where the two of us had to plan out battle strategies together quickly under the pressure of the Active Time Battle System. I enjoyed it. We usually divided control of the characters based on which characters were our personal favorites. Thus, my brother mastered Sabin's Blitz inputs, while I never was able to even remember what they were.
Final Fantasy III was not even the first time an RPG had a co-op feature. Final Fantasy II (now more correctly referred to as Final Fantasy 4) for the SNES accepted input from both controller ports simultaneously. It might have been merely a bug, but it allowed for a similar experience to FFIII's multiplayer, if somewhat more chaotic. Unfortunately, FFIII for the SNES was the last time a traditional console RPG had a multiplayer co-op option. Final Fantasy VII dropped the concept completely, and the multiplayer features were even removed from the Playstation remakes of FFIV and FFVI.
Yet, I still think that asymmetrical co-op in normally single-player RPGs is an interesting concept that is worth taking a second look at. It could be implemented in a similar fashion to what was originally done in FFVI with very little effort. With even more effort, something truly original could be done.
EDIT: A few hours after publishing this, I realized that I had made a critical oversight: there is one RPG series that has supported co-op for mot of its existence: the Tales of... series. As an example, Tales of Symphonia for the Gamecube could support up to four players at once during battle sequences. Since the Tales of... series uses a real time battle system that plays a lot like an action game, there is even a tactical advantage in having a second player controlling one of the characters, rather than an AI. The Tales of... series is an example of how co-op in a genre that traditionally doesn't have multiplayer can make a game or series stand out.
I also think that there is still room to expand the idea of RPG co-op, particularly in the areas of puzzle solving and exploration.