Sometimes the beds have been so bad, and my back hurt so much, that it ruined my trip. And I am pretty young and with no back problems (normally). Other than cost, I have no idea why resorts, condo's, and hotels resort to these horrible matresses ... and then slap them on a slab of CONCRETE! Maybe it is an availability issue.
What we have done if we arrive and the bed is too much for us is to go to Walmart and purchase an air bed. They have a battery operated air pump built in. Did I mention they are CHEAP. OMG
... what a difference they make. Just a little tip.