If left untreated, there is a 1 in 100 chance (1%) that your baby will become infected with GBS (in a GBS+ mom).
That one percent is enough for me to get the test, and get antibiotics if I am positive (I was neg with my daughter, positive with my son).
I am a NICU RN, so I have seen babies with GBS. They get sick FAST. I have seen babies that were healthy in post partum... then suddenly they have a dusky spell and are rushed over to NICU half dead. That is enough to scare me. So I will be getting the GBS swab done (this Thursday I believe) and I will get A/B during labour if I am positive.
Back when I was pregnant with my daughter (14 years ago) the GBS swab was done at 28 weeks. With my son (and still the way it's done for the most part) it's done around 35 weeks. I questioned why it's done at 35 weeks and not 37 or 38 when most practitioners start doing VE. The answer was that too many women end up delivering before the test is done. So they do it at 35 weeks.