In my games, all characters get one automatic 18, the rest we roll dice and let them fall where they may.
This works for my group because none of us want to play Joe Bloggs who'd be better off as a commoner reaping hay. We want to play HEROES, like Sherlock Homes (18+ Int), Conan (18+ Str), Robin Hood (18+ Dex), James Bond (18+ Cha), etc. When we reach the lofty heights of Epic level, we want our characters' exploits retold in taverns across the land, our characters just as famous as luminaries such as Mordenkainen or Elminster.
From a metagaming perspective, I also find it enriches the game. The Rogue is alot more confident when attempting to disable traps, so he's more likely to step up to the plate. The Fighters tend to be more confident in fights, particularly against hordes of smaller enemies, but it doesn't give them so great an advantage that they're not wary of larger monsters. Spellcasters get more spells per day, not to mention access to higher level spells (or does everyone conveniently forget that you need an ability score of 10+level of spell to cast? Why level up as a spellcaster at all if you don't have at least a 19 by the time it becomes important?) increasing their usefulness to the party overall. All in all, everyone feels more satisfied with a day's work, and rightly so, you tend to get more done in a day than a baseline party can and the characters can afford to rest less (I personally have problems with parties who sleep twice in a day... I can't sleep 8 hours twice in a day, and I doubt I could sleep inside the Temple of Elemental Evil at all...).
Finally, the players are more likely to take risks if they actually have a decent chance of pulling it off. I've seen too many players have their spirits of adventure squashed because they tried to swing on a chandelier and failed their Dex roll... if they want to do that stuff, they should have a decent chance of doing it! They're supposed to be the heroes after all!