People seriously have enough trouble with battles in this game that using Osmose occasionally is a serious hindrance to progress? Let's face it, you're not going to be casting Ultima every round except maybe in some of the toughest boss fights, so with 500 MP you probably won't need to recharge MP more than every ten battles or so. Hell, I rarely even find myself casting spells above level two, simply because they're so cost-ineffective above that level in this game (if memory serves, FFV is the only game from this era where higher-level spells are more cost-effective).

Obviously everyone's playing style may vary. I can see the point in using an economizer in a long stretch of grinding, like say on the Veldt, or if you're running a low level game, but in normal combat it's pointless as long as you pay attention to your MP level, which is a trivial task.