If the holster is inherently unsafe due to a design flaw (i.e. it causes discharges) is one thing. Safety is obviously the foundation to any training program. I haven't heard of the Serpa causing weapons to go off, so I don't prohibit their use.
But if somone brings his equipment of choice (weapons, mags, and associated kit) and something fails during class...that's a good thing. At least the failure was discovered during a non-threat situation. I'd want to know if my holster won't work after it gets a little dusty.
Additionally, how often do we change the layout of our gunbelts, holsters, mag pouches, dump bags, etc? I do it all the time in search of the perfect setup. "Discovery Learning" certainly isn't the most academically rigorous approach, but it's pretty darn effective.
I certainly respect anyone's decision to run their business and/or classes in the manner they see fit, so please don't take this as an indictment. As I said earlier, this opinion of the Serpa is fairly common, and my view may be the minority.