I have just skimmed this thread and don't know where it might have been discussed before but I have used a snorkel in the past just to increase my aerobic capacity to work harder in the pool. I think it works great. I'm getting ready for an open water swim so I'm not using it now. I also just swim on my back to accomplish the same thing but give my shoulders a break.
I think, like most tools, they have their place when used "properly" which, in this case, may simply be determined by what your goals are.