Osmose in FFVI is incredibly useful; it pretty much makes ethers and tents unnecessary. I don't think there's a single point in the game after you learn the spell where you can't recover enough MP from monsters using it to make other methods of recovering MP vastly more expensive than they're worth. I never tried it in FFIV mostly because I didn't know what it did when I played the game. (I was a smurfing n00b ok).

I'd say Death is pretty useless because most of the monsters you would want to use it on are immune. Except in FFVI where you can use the Vanish trick, but most of the bosses in that game are so smurfing easy that really, what's the point.