[i am adding in this disclaimer: this post is a rambling mess, because on this issue, I am a rambling mess]
Making the main (or any) character gay would not make them shoot or climb or throw better, no. but that is the point. Being gay doesn't make you better or worse at anything than being straight. If a character was shown to be entirely 'normal and capable' (dare I say it, straight acting) and he turned out to be gay it may make a few of the on the fence bigots out there thing "hey, not all gay people are limp-wristed nancies".
It doesn't have to be (and shouldn't be) shoved down people's throats to be effective either. People say sexuality doesn't come in to most games so why even put gay characters in? I can assure you the main characters (hetero)sexuallity is nearly always referenced and it doesn't just have to be terrible mannequin sex either but it is in nearly every game and it is always hetero and becuase of the world we live in it doesn't even register for most people, Take the GoW2 trailer:
http://www.youtube.com/watch?feature=player_detailpage&v=v1EIaWtGwto#t=18s
Right in the trailer you have the main character looking at a picture of his wife so you know before you even play the game that the main character is straight. Most people in the comments sections of articles about sexuality in games prove that a majority of heterosexuals don't notice the constant affirmations of heterosexuality in games. And fair enough there is no reason they should but you cannot argue they are not there and so any gay references should equally not be there.
Not referencing sexuality at all so a character could be gay is not the same thing either, because as I've already stated it will just be assumed the character is straight, but something as simple as the photo in the trailer would work and you'd have to be a hardened bigot to get your knickers in a twist over that. Problem is that a lot of gamers are hardened bigots. Saying game writers are to poor to be able to tackle it isn't great either - if we can have poor straight characters why do the gay ones to be brilliant and believable? We can't realistically wit until game-writers are any good at it - they just need to write equally badly for a diverse cast.
As to why the world needs gay characters at all, when you are gay and struggling to come to terms with your life being different to everyone else's and you feel alone it does help to have a gay visiblity wherever and in as many different places as you can find it so you know there are other people out there like you and that gay people aren't just the things that are shouted at you in the playground at school. It is perfectly valid to think 'it's only a video game why does it need to try and solve the world's social problems?' and you'd be right. It doesn't, but why the hell shouldn't it?