In my experiences, the wizard didn't have to try. He had to actively avoid it. I always go back to the rogue and wizard example, but I always find it apt. No wizard, even the one with the biggest heart for his fellow thief, is going to pass up invisibility as a spell. It was too good a spell to pass up. For a single 2nd level spell slot:
- you could scout for hours (1 hr/lvl before 3.5) as long as you didn't attack
- you were not forced to stick to shadows or cover & concealment
- you had a 100% chance of not being seen (compared to a thief's % or rogues skill check)
- You could move your full movement rate, even run, without penalty
- You gained bonuses to hit and AC vs foes (though it negated the spell)
- Gave every benefit Hide/Hide in Shadows did (sneak attack, etc)
You can argue invisibility was overpowered for a 2nd level spell. You can make a point that rogues can Hide multiple times a day (whereas wizards were limited to spell slots, forgetting a sorcerer's 6/day for a moment) but you cannot argue a wizard, when he has invisibility prepped, has as good if not better a chance of successfully scouting as a rogue does.
Invisibility is in the PHB. Its been there since 1e (and probably longer). Unless the DM specifically banned it, it was fair game to trample a rogue's scouting role.
That was the problem with most spellcasters: they could do the non-casters job just as easily. A cleric with divine might (PHB) is a fighter for a major combat (and was no slouch unbuffed either). A druid wild-shaped into a bear is as good a warrior as a ranger (and probably a better scout and tracker, thanks to WS/spells). Polymorph opened all sorts of melee monsters up to the otherwise frail wizard (hence the several re-writes and final dissolving of it). These aren't estoric choices out of Complete Munchkin, these were common PHB tactics!
I might have been the odd one out, but I cannot think of a game I played in for long where the spellcasters didn't eventually outstrip the noncasters in importance simply by doing their job.