Hi Robert,
swmbo and I use king size stretchers when tent camping and throw our swags on top. Apart from being off the ground, the benefit we find is that it gives space under each stretcher for storage and easier for our aging bones to get in and out of bed, as opposed to trying to get up off the floor.
We also put an old blanket/sleeping bag on the stretcher, (then the swag on top) to assist with a bit more insulation (warmth) from cold penetrating underneath.
In terms of protecting the tent floor, we bought some furniture castor cups with felt/carpet bottom under each stretcher leg. Hope this helps.