If you want to run YOUR boat while out of the water fine, do it. But suggesting that others do is extremely bad advice.
You cannot dispute the scientific facts of heating up rubber. Heating up the impeller(rubber) will change the molecular properties of the rubber. The thermal and physical effects, as Jim pointed out, will surely weaken it. So if you don't get a complete or partial failure, surely you will get lower water volume/pressure. Just because you can't see any damage does not mean that it hasn't already happened. Why take the chance? Is $30 bucks for a new one each year worth it? Is not changing it and wasting a motor worth it?
I'm not all that smart when it comes to science but it's pretty easy to see that heating up an impeller is never a good thing. Sure you might get away with it a few times, or forever. But at some point it will catch up to you.