That isn't "balance", it's practically the opposite! If the physical stats don't control player volition, neither should the mental ones. You're not "stopping the evil powergamers", you're just saying "Hey everybody, go play a Wizard!"
And no, being poor at bending bars or balancing on a beam isn't the equivalent of not being able to use a plan because you're "too dumb". The equivalent of mechanical penalties is ... mechanical penalties. Which most systems already have. Things like less skill points, bad perception rolls, being worse at a number of skills. You don't need to "double charge" for low mental stats.
And also, 8 Int is not a fricken moron. Some people see "8 Int" and they think "Thog smash! Thog not use tactics! Thog not avoid obviously trapped door!" Give it a rest. Even wolves use flanking and they have Int 2!
This, entirely this. The game has enough built-in punishment for having bad stats in x as it is. I've got a 3.5 cleric right now who's going to be in a world of hurt before long because the houserules in use give Elves -2 STR instead of CON and take heavy armor away from clerics, and the only viable place to put the 9 I rolled was in DEX, ensuring that my character will have terrible AC/Init forever unless I take a couple of feats to fix it/find some gloves of Dexterity.
So derp, there goes my DEX and praise Corellon that I'm an elf so I have a DEX bonus to get it out of the negatives. (Can't dump INT, need the skill points, don't have enough even with 14 INT; can't dump CHA, nobody else really has it; can't dump STR because lol 7 STR + armor + weapons + other crap = Enjoy being at a heavy load forever.) Yeah, there's a penalty to having bad physical stats.
Yeah you know what the penalty for having a character with 8 WIS because it was the only viable place to put it for him as well was? Failing every Will save ever, even with half his levels in a class with a good Will save. Not making spot checks.
Not 'Excuse my while I act permanently out to lunch and a complete moron'. He didn't have an exacting eye for detail and he was a bit impulsive. Wow yeah so mentally deficient and I was totally playing him wrong by not having him poke everything and run around like he was some kind of overgrown five year old with lots of weapons. (Oh wait, maybe it was because he still had 14 INT and 14 CHA.)
(This is why I like 4e's 'put your higher of two stats towards this NAD' system and houseruled it in for saves when I ran 3.5. Yeah my character with great force of personality and confidence who is very sure of himself falls apart at everything. Except wait, no, people like that don't. Yeah my intelligent character is too stupid to anticipate anything and thus get out of the way in time. Yeah my strong character is - OK that one breaks down because I've only ever seen one person dump CON ever because it's far too valuable in every version of D&D ever.)
If people actually *followed* some of the stats you end up with on characters to the degree half the people in this thread are insisting on, we'd all be playing a bunch of utterly psychotic mental patients with massive deficiencies. Now if you rolled like a 3 for INT, OK, sure. If you have an 8... Yeah, no not really. Frankly if you think slavishly playing someone with 8 INT as a complete idiot who never has a good idea, ever, and consequently can't function in the world is good roleplaying, I'm glad you're not in the games I'm in. >_>
(There's an easy way around this, incidentally, in which you use point buy, don't allow any stat below 8 before racial modifiers, and only allow one stat below 10. There, that wasn't so hard, was it? Oh wait, in before someone goes on about how point buy leads to munchkinism and bad roleplay.)