I can't stand it when someone is the only person without a gift at an exchange, so I agree with having extras on hand. I'll never forget the time we all gathered on Christmas Eve at my ex husband's grandmother's house and all of the kids (there were a ton of them, too) received gifts, but someone had brought along a little girl who must have been between 10 and 12 years old, and she received nothing. I know she was hurt and embarassed and I felt sick about it. How can people be so thoughtless!
I like to do this also for the little tiny kids to 'shop' for their moms and dads, since I don't take them real shopping. Anyone is welcome to shop the box (sometimes money is tight, as we all know).
When we have had unexpected guests show up on the evening we are exchanging gifts, I feel bad for anyone who is giftless in the midst of all the gift opening. (If I brought someone along to such an event, I would make sure that I provided a gift for them.) These last minute gifts I designate from Santa and not from me, so there are no recipircal feels planted.