To those who are regular guests at Christmas and never host, just remember to take a substantial amount of stuff as a contribution - ask if advance what you can bring and when the host politely says 'no need, just b ring yourselves' push a little harder and say 'no really, we'd like to bring something - what would be useful - a cheese board, a few bottles of wine, dessert.....' Because if pushed a little, most hosts will gratefully accept an offer. And if youbhaveviffered money because you're a house guest and there for several meals, but that has been turned down, what about sending a supermarket gift card after Christmas £50 or whatever as a thank you. Hosts often say 'no' to offers of help, but I think it's incumbent on those who are regular guests (not necessarily just one off guests) to find a way to give.
And remember again that the £4.50 bottle of wine or the box of Roses isn't a good contribution if you're taking a family of 5 for a full Christmas meal or several days of 3 meals a day.
Fine not to contribute much if you're a one off guest or having a hard time and the hosts are genuinely happy to foot the bill, and fine for families to be honest with each other and those with more to support those with less without any sense of it being wrong to say or do it - isn't that what families should do for each other - but if you're doing okay and find yourselves regularly at someone else's for Christmas perhaps due to geography or house size or whatever and only make a mean contribution or yet the years pass without finding a way to contribute properly, I think that's pretty tight and selfish behaviour.
Contributing can take many forms too - you can take desserts or crackers or booze that's been asked for, or hasn't been asked for. You can send a gift card for a supermarket in advance or after the event. You can say you'll bring items for the Christmas tea or you'll get a takeaway for everyone on Christmas Eve or Bosing Day, or take people out for a pub lunch or a breakfast if you're there for a few days, or you can order the Boxing Day Ham or sausages or whatever you have, or a turkey, or a nice Christmas bouquet. There are loads of ways to contribute.
No problem if everyone takes turns or when there's just one extra, but when you're talking about whole families regularly receiving hospitality with zero serious effort (and a half hearted 'what can I bring' doesn't count as real effort) to contribute in terms of time, effort, money or contributions, I think it's pretty poor. Especially when the hosts are themselves wither not especially well off or are getting on in terms of age. Too often adults behave like they've returned to childhood at Christmas and seem to think Father Christmas magics everything up. He doesn't.