Remathilis
Legend
It might be interesting if many of the players who prefer 4E's focus on game balance played mainly bought adventures (which usually had many many combat encounters).
Adventures serve as a great example; they assume a certain power curve (by nature, they need to write to a baseline) but it goes further than that.
3.5 assumes 13 "combat worthy" encounters of equal challenge rating to advance a level. The encounters could be negotiations, sneakery, or melee, but they are, in essence, as difficult as a combat with an equal foe. This is all the core rules reward.
The game assumes that X level PC can handle X level challenge. An 18th level PC (with his party) can face a CR 18 monster, and live.
The problem with sub-optimal choices is that the PC =/= to his level. Our 10/10 fighter/wizard (or sub in any poor choice combo) doesn't take blows like a fighter nor deal damage like a wizard could in those situations. When dealing with each role (fighting, casting) he's little better than a PC 1/2 his level. And due to the economy of actions (one standard action a round) he doesn't even count for two PCs. His vaunted versatility is useless because he sucks in both roles.
(Prove my point: create a party of 10th level single-classed PCs. Give them level 20 treasure and double their hp. Now, let them fight a Balor. See if they win).
So the SYSTEM ITSELF (and its inherent assumptions) begin to break down. The PC is =/= to his level, so challenges equal to his level are actually harder than anticipated. He cannot penetrate monster defenses. He cannot locate or disarm traps. He cannot remove/negate the status ailments monsters produce.
So the DM has to compensate. Weaker monsters. Weaker traps. Dice Fudging. The CR/EL goes off the rails. You need more weaker encounters than 13 to level. Appropriate challenges slowly increase the chance of TPK.
Basically, the system and all its balancing assumptions goes out the window.
Modules show this glaring weakness best by being neutral. The DM has to let the dice fall where they may, or seriously alter them (and thus show how much he must deviate from baseline assumptions) to make these sub-optimal PCs work.
If you are the type of DM who either a.) tolerates rampant PC death OR b.) uses a nontraditional setup to insure the game remains "balanced" then this is a non-issue. You have gone a fixed the issue your own way. However, the core rules of the game shouldn't require such patches to work just because of booby-trap PCs. There is no warning that "even multi-classing may make your PC less effective" in the PHB, no "DMs must do this to compensate for floundering PCs" in the DMG. It assumes that such a PC is equal to the rest. That is not true.
I guess in essence, my point is that a chain is only as strong as its weakest link. Such characters are the weak link, and it then falls to others (other PCs, the DM) to compensate for that weak link somehow. I much prefer a system that doesn't allow for so many weak links, even at the expense of "customizing" because knowing my fellow players can do their job well makes me a better player and DM.