Either way you're going to have to hold onto the mask when clearing to avoid exhaling your mask off your face. Well not really off your face, but enough to make a watertight seal impossible. Having a purge valve doesn't make hand-less mask clears possible.
So with that in mind I say purge masks are one of those unneeded gimmicks; though I have never used one personally. They probably make mask clearing easier but they tend to add problems of their own that you personally may or may not want to deal with.
The nose pocket will tend to be hard plastic or in such a way that the purge may rub up on your nose.
You may also have a problem with forcefully over clearing, forcing air through your purge instead of occupying your mask and forcing water out.
This is the same problem with purge snorkels in which too forceful an exhale can leave you with a snorkel still full of water.
With non-purge snorkels and masks, this is less likely or even a non-existent case.
Sand and grit can also ruin the purge, leading to an ever flooding mask until the purge is cleared of said grit.
In retrospect when you clear your mask you create a purge valve of your own between your top lip and the mask skirt. One finger on the top frame of your mask, exhaling lightly, and then tilting your head up, is often all you need to clear your mask (provided a proper fit and seal).
You have to do this with both purge and non-purge equipped masks.
It's easy enough, so IMO the purge valve mask only has one real advantage, when your face is above the water, any water in the mask will automatically drain out the nose pocket.
