We moved a twin mattress into Pumpkins room the other day. We did this with Peanut about the same age; it's a nice gentle way to get ready for a regular bed. Of course, Peanut started sleeping in hers right away. No chance we are letting Pumpkin do that - he is too adventurous and climbing a bookcase or something in the middle of the night is a very real possibility.
So he can choose to sleep there for naps, and he has taken one in the big bed so far.
The best part about having a mattress on the floor in your room though has nothing to do with sleeping. It's all about being a trampoline!