I agree with the posters saying the effectiveness of BC is, first and foremost, really hard to measure. Especially the kind of BC that allows a save. Glitterdust and Stinking Cloud keep popping up, but they're actually worse than a Fireball if the other guys make their saves. A "smart f'ing idiot" will shove that in your face immediately. If you really want to convince somebody, make sure of the kind of examples you use.
I present to you: a way to judge what's an effective spell and what's not (rough draft):
Good, inarguable BC: Obscuring Mist, Solid Fog, Wall of Force, Dimensional Shuffle.
(These will work no matter what, making them superior to everything else you could do generally)
Shaky, circumstantial or save-dependent BC: Grease, Glitterdust, Stinking Cloud, Black Tentacles, Major Image.
(Most of these are superior to Fireball, but only because they have multiple effects, or their effect when they do work is just freakin' awesome. Depending on circumstance and success, it may be better to use these over the aforementioned)
Mediocre all the way, but stackable with what the rest of your party is doing all the time: direct damage spells.
(A fallback option no caster should be without. Sometimes it's you who must do the damage, when the other guys are occupied/dead/not there. Sometimes it's just better to take out that dragon with only 5 HP left now. Direct damage will work. It's just not the most economical way to play to your strengths as a caster)
Special mention: Save-or-die, or save-or-suck.
(It's really hard to judge these on the basis of effectiveness alone. You either do it all, or you do nothing. Normally, I'd leave these alone, if only because even if you succeed, you won't feel good about it, it's just anticlimax)
If your friend the f'ing idiot played blaster mages all the way since 1st edition, he'll always fall back on direct damage. Even if you tell him just how many HP most creatures in 3.5 have. If he's used to winning fights because you reduced the monsters to 0 HP, he'll be having a hard time learning differently. Coz you know, it takes imagination to do that. Good luck anyway...
EDIT: Wow, this had a second page I hadn't read before posting. Sorry if this was a little late in coming, then...