I really depends on the people you are inviting or givin a free vacation.
DW and I gave a 1 week stay in NYC at Hilton to some of our friends, they were so grateful. Even though they are good friends we didn't know that going to NYC had been a life long dream of theirs.
So when we asked if they wanted go, they were so happy - they accepted and had the best time ever.
IMO if I give a free vacation to anyone I expect some sort of gratefulness - I also ask people to take a real hard look in their calenders to see if they are actually able to go or not. Dont accept if you don't know if you can go or not.
I you give a vacation away then it should be free to the receiver, but if you plan a vacation with someone else I think its more than fair to split the cost of the dues or whatever the price of the vacation is - you just need to mention it on beforehand so everyone ones. Let them pay their half on the time of the booking - as others mentioned its a commitment.
I haven't been "burned" yet, should it ever happen I might revise my thinking.
I haven't travelled with friends...yet, But should it happen I would have a talk about expectations and so. I would hate for a vacation to be ruined because of this. I find it more than fair if the owner gets the master bedroom - afterall being an owner does come with some upfront costs.