I have mixed feelings about women's only events: I think they do serve a purpose in addressing an imbalance in sports in general, and running in this particular case, where women are more likely to feel intimidated by events that seem more competitive, serious, or male-dominated. I think it would be hard to deny either that more men than women take amateur running 'seriously', or that men, in general, tend to run faster. I am not intimidated by male runners at all (though I do wish they would be more careful where they spit during races, it's always the blokes...
) , but when I wanted to join a club, I did e-mail and ask if they had any women members (they were quite proud to be able to reply that they had four!), and even I was a little put off by the thought that I might be the only woman. So I can imagine if I were less good at running, less sure of myself, didn't already do races, etc., a women-only event might be a good way in.
However, I do sometimes feel like these mass-participation neon-pink coloured events do tend to reinforce this perceived divide between serious running and women's sport, which is branded more as a kind of girly keep-fit activity and I wonder if it actually does more to reinforce the idea that women can't expect to be competitive or serious about sport. You see it in all the sports shops too, where the men have serious, useful gear, and the women get lots of neon pink trim and no bloody pockets. Grr.
I agree though that there doesn't seem to be any good reason for men-only events. It seems like a sort of 'well, if you're going to exclude us from YOUR races' reaction that completely sidesteps any of the actual arguments for or against.