I don't really like recycling game content/campaigns with different characters. Most of the time it's just done wrong. The stories and sequences of events may be different, but you're still playing the same game tailored for all the characters, rather than a single game for each of them. Odin Sphere is a beautiful game, but I don't appreciate the overall package because for each character, it just feels like I'm playing the same game but watching a slightly different play/movie. I might like the characters, but I'd rather a game have a single campaign, with similar characters, that's highly replayable rather than several similar campaigns with different characters. The closure when I beat one campaign feels less rewarding when I know there are 3 or 4 more similar (recycled) ones I have to beat in order to get the "real" ending*.
This is partly why I like comparing ZX to Metroid (or MMX) because they're both similar games, but I think the latter does powerups and game progression much better. The map and the progression are scripted in such a way that you rarely ever feel lost, but still have fun exploring the vast non-linear world as you progress from area to area; with MMX, the stages are disjoint but very well designed, and the whole experience is cohesive anyways thanks to some light story injection and a cool stage select screen. The powerups you collect in both games stack up rather than provide you a range of mutually-exclusive forms/armors that you probably won't end up exploring or enjoying as much as a single upgradeable form, because the game isn't designed in such a way to make each form feel useful and rewarding**.
In ZX, you're occasionally forced to use one of four specific forms because of a roadblock, but that's not adequate to make the form either fun or useful. ZXA jacked up this useless quotient 3 times over by allowing you to copy boss forms--all 8 of them--as well as the previous four Guardian and ZX forms. In both cases, the game is designed for all the different forms, making the gameplay more complicated and less streamlined that it should be. Had the game made each form's usefulness more natural, they would've been more fun and worthwhile to use, but it also would've meant a much bigger game that demanded a departure from the 8-boss, 8-stage progression paradigm. ZX would have to have been more like Metroid than it was Mega Man, which is admittedly something I want to see.
I didn't mind it much with X4 when you had to choose between X and Zero at the start, but I think it would've been a slightly smarter design choice to reward the player with Zero's campaign for beating X's campaign. So we both agree that unlockables are a good thing, because they're rewarding. Most of the time. On the other hand, having different characters with different game campaigns that intersect with each other is actually not a bad idea. Breath of Fire IV is a good example of this.
Also it's a huge [sonic slicer] to grind for money to fix the forms after you beat them out of the boss you fight. Why are they broken? Because you kept hitting them in their weak points, which you'd naturally think is a good thing (after all, you get a shiny "Level 1 Victory" for it!), but it actually means you have to pay more for that piece of your form to not suck; what you really wanted was a "Level 4 Victory". Did I mention this was really unintuitive?