I completely agree. At our wedding, the invitations were for couples, single people plus guest, or families with children. I could not imagine excluding children from our wedding nor could I forbid single people from bringing a guest.I definitely agree it is VERY poor taste to add on invites, but I don't think it is proper to invite a single-ton and not allow him or her to bring a guest. She probably doesn't yet know who she will be seated with, right? Isn't this something you find out when you enter the reception?
I was once invited to a wedding but warned by the bride's sister prior to receiving the invite that I might not be allowed to bring a guest because the count was getting high. I, of course, never said anything, but found this quite rude. The invite did include guest, so we went. If it was just for me I would have declined.
There are so many different friends and family groups and "coupling" at weddings (who do you slow dance with if you are by yourself) and the bride and groom do so much hopping between guests, I think it is only right to allow everyone (married or not) to bring a guest so that they have an enjoyable evening too.
I'm really surprised that there are people in this thread who know exactly how many people were at their reception. We had a few hundred people at ours. I have no clue exactly how many people there were or whether specific RSVPers actually showed (outside of the 50 or 60 'core' group of friends and family). I guess that maybe my wife is somehow aware of this information, but I rather doubt it.
For the caterer, I would take the number of RSVPers and add one or two percent. Between non-RSVPers, random guests and no-shows, you'll probably be very close.