I think this is totally "know your audience" and "depends on the situation".
I'm a "the more the merrior" kind of host, and my friends all know that. So I've often gotten asked if it's ok to bring someone additional which is fine. Or suggested it myself if, for example, someone has a friend/relative visiting from out of town during the date of the party.
However, I would not be amenable to extra guests if it was a seated dinner party where I've got limited space at the table. So even I, who would almost always welcome extras with open arms, would have to say no sometimes, and in those cases, rare though they are, I'd prefer not to be put on the spot in the first place. In that kind of case, I'd rather invite the single alone, or ask if they wanted to bring a guest when I made the invitation.
So not only do you have to take into account the host you are asking, but also what the specific circumstance of the event is.