I'm not sure that it should be a routine test if I'm honest. I believe it should be offered to women if they are worried about it, and women should be informed of the risks of both treating and not treating, diagnosing and not diagnosing.
The US have been the trailblazers on the GBS issue, it is a routine test over there, and routinely 'treated' with antibiotics during labour. And as a pp said, they have seen no improvement in the transmission rates or of the mortality rates of those babies affected. It is important to remember that prophylatic treatment for GBS is not benign. Antibiotic usage and overusage should not be taken lightly. Here is a good article about some of the problems encountered in the US due to routine antibiotic administration during labour.
I totally support treatment for women with known risk factors (PROM, prematurity, etc) or who are worried about the issue. It is something else that should be a choice. I think that test should be offered just as antenatal screenings are - most low risk women will not opt for invasive testing such as amnio, for example. Those women should, I believe, have the OPTION of abx in labour.
But the test is imprecise, the treatment confining and could cause futher problems, personal and societal, and the outcomes unfortunately not dramatically improved.
I don't wish to marginalise anyone with personal experience of GBS, it is a major problem in a newborn, and I feel for anyone who had to go through the trauma of losing their child to GBS, or having their baby in the NICU.
Blanket testing and treatment, though? It may not be the answer...