When I first joined I did my best to reply to every PM, but it got a bit out of hand and I just couldn't keep up with them all, so I ended up cherry-picking them and answering the ones that actually interested me.
You don't want to be rude, but you also have a life outside of BM!
(Mind you - sometimes they just get blanked because they've obviously just taken the scatter gun approach to messaging wimmen in the hopes that someone takes them up on it, rather than actually bothering to read your profile!)
When I arrange an event, I'll often drop PMs to people who've signed up for it that I've not previously met but it really doesn't bother me if they reply or not. I don't see it as being rude - I've just given them some information to help with turning up or how best to find us on arrival. It's not necessary to reply.