I'm definitely not a fan of those sorts of characters. I do think there's a line that is crossed in optimisation, I just don't feel that I cross it.
I think the line should be: a PC should not exist that is more powerful than what you can build from the players handbook.
That's the baseline. Because in just about every edition, the combos that could be built from the PH are usually balanced.
When you can go shopping through all the add-ons for parts, thats where you find testing was weak, and opportunity for imbalance.
I play 3x, so a lot of the stuff Kzach is talking about isn't available. But the concept still applies.
If you build a fighter and don't put your best stat in STR, and all your skills rely on your worst stat, that's goes against most common wisdom since the inception of the game. sure, there's the exception (like using DEX and feats that make it so your DEX bonus is used in melee). But the principle remains that you put your stats where your class needs it, and you generally buy skills that use your best stats.
Let's call that another baseline assumption.
Pre 3e. that was the only choices to make. Pretty hard to screw up OR to make a better fighter than the fighter I made.
Post 3e, feats, PrCs, and all the other stuff (4e has new stuff I'm not familiar with) put in so many extra variables that you need lots of time, or a program to make it easy to test and swap what your stats look like.
But I think it still comes down to with those, if you can build something extremely better than what the PH allows, then it violated the design specification. That is the design standard for balance because it is the first product that sets the scale for what is possible in the game.
Table cultural is the last leg of the problem. Most of the people I game with build their PC 30 minutes before the game (or the night before) on a piece of paper. When we level up, we flip open the PH and find what new feat/skills we want. We don't plan these things out, barring occasions where there's a higher level thing we want that has pre-requisites. But you won't see a 20 level build plan.
Hopefully, our PCs don't totally suck, but anybody who puts in more time on planning out, and using more books than the PH could probably find more powerful combinations. However, if it violates the first baseline, thats a sign of imbalance and exploiting that imbalance.