Well I think that those who don't think it's weird are weird!
I don't care what other people do, I just have an opinion on it, like with most other Mumsnet matters. I might not care if someone breastfeeds their baby or not, but I do have an opinion that I can express.
I don't care if your kids get cards from other kids, or even if they want to make you a card. I just find the idea of parents sending their kids Valentines cards a bit odd. If it was supposedly from some secret admirer then fair enough, but obviously from mummy or daddy then it's just that, odd.
I don't do Valentines Day either. I know we can bypass the commercialism and make our own cards etc, but why would I want to declare my love for dh on a day when everyone else is doing the same? We have our wedding anniversary, that's our special day, and we don't share that with quite so many people.
So you see, I don't really care what you do, but I am entitled to think you all weird for doing so!