I'm certainly no expert on training at altitude (although I have climbed a number of peaks over 20,000')
But I thought that some of the effects of training at altitude are due to the lower partial pressure of O2 at altitude resulting in less available oxygen molecules. The training effect as I understand it, comes from an increase in red blood cell volume and an increase in EPO. This comes from being continuously in a low oxygen environment over an extended period of time. In using the training mask I think that you are essentially breathing the same oxygen rich air, plus one typically does not live wearing one of these things.
Again, I am no scientist so it would be great to hear some thoughts on the science behind altitude training. I would certainly welcome learning more on this topic, and possibly clear up any misunderstandings I may have about it.