Well come on now, Zelgadis is awesome regardless of anyone else's spell repertoire. Being the jack of all trades that he is makes him much more useful overall than anyone other than Lina. While there are plenty of situations where Zel can't be the shining hero saving the day I can't think of a single situation where he was a liability in battle.

Zel is a support character and that's just the way it has to be, but he's one of the better support characters out there. He can be relied on to take care of himself in pretty much any situation, unlike Amelia, and to some extend Goury, who sometimes have to be saved from something immune to their area of expertise that's a non-boss. He may not be the heavy hitter with magic that Lina is or with swordplay that Goury is but if you need someone at your back and you have no clue wtrf is around the next corner Zelgadis is definitely your man.