I'm not going to say Illusionists were worthless in AD&D 1e - they weren't. They had their uses, but they were very specific and fairly rare. They were, however, a very poor substitute for a magic-user in the party, and if the party passed on the mage for an illusionists, especially at low levels, they were pretty much screwing themselves. (Unearthed Arcana changed this balance a bit and made the Illusionist more viable, but it was likely he would still be the second "arcane" caster i0n the party, not the only one.)
Gnome thief / illusionist made for a decent class combination - it's just a shame that folks in my groups always imagined the gnome in D&D to look like that stupid garden gnomes - as a race, gnomes never seemed to have a niche besides "talk with burrowing animals", but that's a whole 'nother post.
The fighter subclasses could easily substitute for a regular fighter, but it wasn't the same for the magic-user and it's subclass (and to some extent, that probably applies to the cleric and it's druid subclass too).
So, how much use did / do Illusionists get in your campaigns?