In most AD&D campaigns I played, no one wanted to play the Thief. It's just not FUN to be sitting there making percentage rolls while the other players twiddle their thumbs. Same went for the Cleric; no one wanted to be a walking Band-Aid when they could be something flashier.
So, the DMs had only a few options. Offer "incentives" to play those classes (an extra level for the thief!); provide an NPC; or, worst case, design the adventures such that the lack of a certain class (thief) wouldn't cripple the party.
Fast forward to 3E. Being a Cleric is pretty good, thanks to the domains. Being a Rogue is pretty good, thanks to Sneak Attack and Tumble. But now, it seems that no one wants to be the "front man", the CHA-centric character who takes all those social skills. They either want to be able to "roleplay" every NPC encounter in a way that negates their characters' low CHA/skills, or simply do without those sorts of checks.
Bottom line: don't let them get away with it. If the party is consistently running into encounters that require diplomacy and finesse, don't let the CHA 8 Fighter do the talking. Or if he does, make sure that the players fail spectacularly, and leave no doubt whose fault it was. In the core rules, the only classes that can really excel at social skills are the Bard and Rogue, and the Bard definitely is better at it. If the players have reason to believe these roles will be absolutely needed by the party, they'll value them more.
The Bard is a fine class, as-is. It can fight, it has plenty of skills, it can cast mass haste and it can heal in a pinch. The spellcasting is spontaneous (low maintenance!), and covers a wide range of useful abilities. The class even gets a few nice non-spell abilities (the songs).
It's almost the only core class my friends and I have never heavily modified (we left the Fighter alone); the only changes we've ever made:
> We tweaked the spell list (remove the healing spells and add some extra protection from whatever spells to compensate)
> We made the "cast in light armor" change before 3.5E did.
> And finally, we changed the spellcasting bonus; instead of high CHA giving extra spells per day, we changed it to extra spells KNOWN. (Only permanent sources of CHA counted for this, though.)