Tbh I was always of the opinion that at airsoft ranges camo isn't going to make all that much difference.
Sure it helps, but ultimately you'll give yourself away by moving, as the eye notices it far more readily.
Very few people, camo or not, are that great at using it properly.
The only reason I got camo was to go for a more authentic look, because as Nikona said, airsoft is about the realism. If you're not fussed, then play paintball instead.
There's also the point Cheese brought up about the hardwearingness of camo gear made to military spec. A pair of jeans will be ruined by mud from one woodland game if you get properly stuck in, so camo is actualy cost effective, as well as beneficial to you visible image when things get tense and you're trying to stay low, as well as your perceived image. As people have said, you will almost certainly be marginalised for wearing casual street clothing to a skirmish, especially trainers.
However, every site I've ever been to have had woodland camo jumpsuits available to hire, so you could just stick one of them on, might cost you an extra £5 a day to hire one though, so again, having your own is just more cost effective.