@ Atomic dog
I build things not as a munchkin, but so that I can stay relevant against the level 20 caster, having a 4 in con, for roleplay reasons is not a good reason. There is two curves in this game that matters, Roleplay, and Usable , those two need to match against each other for all folks int he group to enjoy it. If your wizard can fireball everything before a warrior can draw a sword, true that may be the DM fault for not balancing things.
Certain class's suck when compared to others, I will go with an easy one, the samurai , no one uses it, folks rather take a level 20 fighter and level that , then use that class. Would you as a DM force them to use the samurai instead of the fighter to rp it? At the same time gish builds are some of the usual character arches of Magic+Sword , you know typical elf stuff. Before player handbook two came out, you should have seen how convolted some of these builds are.
Your Job as the DM is to make sure we don't break the power level of the game, not to say, I hate multiclassing so we are not going to do it.