It's up to them, really. Lots of variables like suitability of the venue (catering, safety, how easy it is to get accommodation if needed) down to money (small budget, less heads), or if relatives kids have been little angels in the past, or if they remember last year when Aunt Jinny's kid screamed all the way through that funeral 
So I'd say it depends on the wedding party.
We've been invited to weddings like this before. We couldn't go to one of them as no babysitter, but no harm, we just sent them our best wishes and figured that there are so many conflicting issues when planning a wedding, who am i to judge? but we've also been to 2 that were no kids - one where someone turned up with a child anyway (
slightly rude) and it was a small intimate wedding, so it stuck out, no apology from the parents that I could tell (i was bridesmaid) like an ill babysitter or childminder let them down... which i thought was totally wrong.... and then another one where our neighbours just babysat for the night. it was fine.
So.... to answer the OP... weddings where children are banned are usually done for a good reason IMHO, although it depends on the couple as to what it actually is. You've got to be pretty brave to face friends and relatives, the path of least resistance is to just invite them, so i think guests should respect the couples wishes.
besides, it's a wonderful chance to bond with DH at these events anyway - many parents find it hard to justify nights out together, we do with our finances, weddings are a good night for just letting ourselves be husband and wife again rather that mum and dad 