Propping up "Terrible" Builds

Continuation from my first post, but this one should be fine on its own too.

Let's say I'm setting up for a new campaign, with three of my usual players (the kind of people who build amazingly powerful characters that make my jaw drop and my encounter level tables go tearfully on strike) and one new guy (who I just invited to the table because he saved my dog from a cement mixer and has a cool haircut). We sit down to talk character creation. The veterans are all doing the usual tricks: we have a dervish dance magus, a witch with ability focus: slumber and 22 int, and a halfling oracle of life. New guy really wants to play an agile and lethal fighter who wears chainmail and throws knives. 

Oh new guy... there are so many things wrong with this plan.

If he familiarized himself with the rules, that kind of build would never even occur to him (which is too bad, but that's a story for another post). Nevertheless, that's what he wants to build. As a GM, my options are:

a) convince new guy that what he wants isn't mechanically viable and he needs to think of something else (take away his fun)

b) convince the others that they need to build something equally sub-optimal (take away their fun)

c) scour the boards and sourcebooks in an attempt to optimize a knife thrower fighter (effectively taking over the new guy's character)

d) Homebrew!

I look at what this knife thrower is bringing to the party- he's a close range, damage-oriented attacker with 1/1 BAB. That sounds to me like a gunslinger. So I go build a pistolero gunslinger from level 1-20 and see what his damage output looks like. If the new guy had built the optimal build for his chosen role, that's what he'd be doing, so the way I see it, that's about what he deserves to be doing to fill that party role in a group of players who are otherwise Hard, Optimized Dudes.

So what makes the gunslinger the better choice? He only needs to keep one weapon enchanted- he can make all his attacks in a round with only one weapon, and can even bypass damage reduction using silver or cold iron bullets. Our knifeman has neither of these advantages, and he also doesn't get the gunslinger's abilities to attack touch AC or add his Dex modifier to damage, or his ridiculous no-save high level grit effects. There's a reason a gunslinger is a viable build and a knifeman is not.

I could come up with archetypes and conditions, trade out armor training and bravery for some grit variant. But I'd rather the new player simply play a fighter. Really the only problem is that he will hit less often and do less damage than the 'slinger. So here's my fix: I let the fighter take a feat, we simply call it "Bullseye", that lets him add his strength and his dexterity to both attack and damage rolls with his knives (prereq- precise shot, point blank shot). And the crowd goes wild! Broken! Ridiculous! Overpowered!

Yes. It is all of those things. Fortunately, it is not going into a sourcebook. It is a one-time, character specific event. And it might help the player have a good time at a table of characters who push the rules to the very edge of broken. It might actually bring the group together as the rest of the players, the experts who know how to milk the most bonus from every die, get together to teach the new player all the wonderful ways he can break the game using this custom rule.

Pathfinder is a game of rules that are ostensibly balanced. Realistically, there is no way the game can be perfectly designed to maximize fun for all players at all levels of play. It's easy to forget in such a rules-heavy game that the people around the table can change and add whatever they like to make the game what they want or need for a particular character or scenario to be fun.