You used anecdotal evidence before to put your point across. I personally know women who have been raped by strangers. One is too many - but I know more than that. I have personally experienced dodgy behaviour by taxi drivers on multiple occasions. If I chose to catch a female only cab, why would it matter to anyone? I just don't get it.
I made an official complaint re the taxi driver who tried to run us off the road, had his company and license number. They were totally ridiculous, I was chasing it up for about 4 months and kept getting the run around until I finally gave up. I have no faith in taxi companies at all, why would they suddenly just turn around and change their entire culture? Maybe an all female cab will equal a bit of competition and they will have to pull up their socks. I can't see how that could be considered a bad thing.
I get why you don't think it is a good idea, but they haven't done anything, they haven't changed anything, society has a huge way to go, in the meantime, maybe there could be this for those who feel more comfortable. It doesn't mean people have to use it, and I am sure there would be lots of people who don't care either way.