GBS is changeable. You can go from positive to negative and back again in the space of a few weeks, so only testing in labour is really all that reliable. It can make you more prone to interventions (IV drip, concern about prolonged rupture of membranes even above and beyond their normal worry about that sort of thing). And if you have a strain that is antibiotic resistant, as a lot of them seem to be these days, then even getting antibiotics in labour won't fix anything.
In the UK from what I understand (and from what my doctor told me 2 years ago when I refused it with my second), they don't even test for it unless you're meeting criteria that makes infection of the baby likely- such as prolonged rupture of membranes or preterm labour- and the chances of bad stuff happening when your water breaks during labour or with a full term baby is less than the risk of nasty stuff happening from antibiotics (ie, thrush) so he had no problem at all with my refusing the testing, and even gave me more information on why I was right to refuse (kind of baffled me! This was my first appointment with him, and my previous doctor had been on the guilt-wagon for every test that I decided against).
I think I'll refuse again this time, too. If for some weird reason my water breaks well in advance of labour (didn't happen the last two times, but anything is possible!) I'll go for a swab. They can get results fast if they're motivated :)