I can't see many parents WANTING their adult kids living at home with them. Sounds like the parents let them live there cheap so they could save up and get their own place and if it were one of my kids, I'd be a little put off that they were spending the money on a vacation instead of putting it toward the goal of getting their own place.
Most of my friends have have young adult kids and there isn't one of us that would prefer our kids stay dependant on us and live in our homes, the ones that do have kids living at home out of necessity are looking forward to the day when the kids can get their own place. My son is on his own, but my 22 year old daughter is living at home while she pays her school bills. I would never let her boyfriend move in with us and as much as I love my daughter I will happy when she moves out, preferably next door, ha ha.
We seem to live in an entitled society. I actually have heard people say that they think grandparents should be their daycare for their grandchildren, like it is expected when grandma retires that she should now watch her grandchildren while the parents work. If thats what grandma wants, fine, but I also don't know many people that would prefer that. They might prefer that to the kid going to daycare, but I can guarantee that grandma would rather see the kid for visits than have them 5 days a week like another job. Once again she is tied down.
As a mother, we love our kids, but we also want to see them making it on their own. Even though you think you are no problem, it is hard living under a roof with another adult, add in a spouse and kids and it's even harder. If my son were to come back home with his wife and daughter, I would not be thrilled. I would feel bad for him that he even had to, but I would mourn my loss of privacy. I would never ever put them out on the street, and I would make them feel welcome, but my ultimate goal would be for them to get it together and move out again.