Holding the valve down to empty all gas is not exactly the same as firing the gun. The gun uses only a small shot of gas to expel the bb. Using it as a dump valve on a regular basis risks destroying the oring by repeated freezing. Rapid cycling while firing is practically the same thing however.
When and if liquid gas touches the oring it will freeze it. Do it enough times and it will lose its elasticity. I think in the real world this theoretical problem is actually less onerous than it would appear. It is a good idea to leave your gas in the mag during storage. So in most cases dumping gas would not have to be done. When switching gas, wind953, you probably can safely dump the gas with a couple of quick pushes. Leave whatever gas you have in it for storage, and come up with a system (color tag or something) to tell you later which gas is in the mag.