Hmmm....
Well, part of the issue is, even if mom is GBS+, baby is not that likely to pick up the disease and get sick from it (the rate is about 6% according to an article in the mothering.com archives). Then there's an even smaller percentage of those babies who actually suffer permanent harm or death from the disease. Some babies are more at risk than others; although I won't consider doing antibiotics again for a normal, full-term delivery, if I were delivering prematurely I would almost certainly do them.
Then there's the fact that there ARE other protocols besides systemic antibiotics that can remove or lessen the threat, such as hibiclens rinses, garlic, or probiotics.
As to what probiotics are and how they help... the bacterial strains in probiotics are the same as or substantially similar to those found natively in the digestive systems of healthy humans. When the "good bugs" get out of balance, several things can happen. One of those things is that, with the ecological niche vacated by those "good bugs," "bad bugs" can become overrepresented. EVERYONE has a leeetle bit of GBS in their bodies... along with streptococcus, e. coli, and other scary sounding things. ;-) When we get an infection from these bugs, it's because they overgrow and/or get into parts of our bodies where they don't belong and aren't kept in check.
The GBS culture practically always finds GBS; but only 30-40% of women have it at a level that is considered "dangerous." Those women may lack sufficient numbers of certain "good bugs" that help keep GBS in check. So taking probiotics or using probiotic vaginal suppositories containing those particular strains can bring down the GBS count to a "safe" level.
If you deliver in a US hospital and decline the GBS test, the standard of care is to keep you and baby for observation for at least 48 hours after birth. If either of you develop a fever, they'll want you to take antibiotics for GBS infection.