I have let my players pick their scores for some 6+ years and I have really not had any ridiculous scores. Sometimes a guy will make a 50 point buy or something but when that happens the players sort of balance it out among themselves.
They will tolerate one guy taking center for a scenario or campaign then let someone else do it next time. My players are about ideas, sometimes an idea might require those scores. However, next time the player who got to be the hero will take his turn with the cleric or bard...support, support, support.
We don't play with the theory that all 12's and no gear makes for a more challenging game. Its the flavor that matters, a game with power house characters can be just as challenging -provided one knows how to balance the challenges to the characters abilities. Reading straight out of the MM will not be challenging for 30+ point buy characters.
Overly restrictive choices and limits turn players off. Let them play what they want and you as the DM do the work to make the challenges appropriate. Let them work out their party balance issues themselves with roles and such. It has always worked out in the wash for me and my crew.
Hell, I found that the sheer peer pressure put on each other to be realistic has been more of a controlling factor than any - DM imposed balance system.
I guess if you have pick up games and rotate players a lot it might require some intervention but IMG we've had only 4 new guys in that same time frame (6+ years) so even though sometimes the new guy will overreach the first time he makes a new character he will soon realize that he has sort of imbalanced the game a bit.
I have even had a player work with me to make some in game reason for his character to loose ability scores permanently because he felt bad about making some choices he thought were a bit off.
DH -