What strikes me is that Christmas is a unique occasion because all the shops, cafes and leisure centres are shut. If you are, for some reason, alone on your birthday or Valentine's Day or a royal wedding, or Bonfire Night or some other festive getting-together day, you can go and do something you really want to do, like having a swim and sauna. Christmas Day is different. Everything is geared around that one family meal, and, annoyingly, people always ask you, from November onwards, what you will be doing. That's why invitations can get given out out of a sense of doing the right thing.
But I've personally never even heard the waifs and strays expression used about guests, so it can't be that ubiquitous. And some hosts might just be using it, when talking about their Christmas, in a short-hand way, to describe any guest who isn't related so that they don't have to go into a rigmarole of explaining the ins and outs of how they know everyone. Some people are boring with language and can never think of alternative and meaning-enhancing ways of saying a thing, but instead talk in cliches. (Ha ha, it always has to be OH and DP on mumsnet, lol!)
Some hosts might feel bountiful, but I'd say ignore that fact, and turn Christmas Day into a chance to do something positive for somebody at the table who might be related to the host but who maybe is feeling uncomfortable because they don't get on with an in-law, or something, and use the day as an opportunity to make it pleasant for them. A guest might, in a jocular way, be a waif to the host, but to her niece they might be the only person in the house she wants to talk to.