Quite a few trainers are not allowing Serpas. It is quite easy to say that here is a rule and if it is followed then you won't have trouble.
But that is the law and order mentality of the gun world for some and flies against the reality of human factors research. A holster might be prone to screw ups do to operator failure to follow the rule. Rant that they should, sensei master - but that doesn't always happen. Stress, undertraining, etc. - do lead to accidents and a trainer doesn't need to stop a class or stop a match to watch the Life-flight heliocopter arrive.
So it's ideological position against the real world of human performance. In almost every area where human factors cause mishaps - someone says follow the rules, blah, blah - however if the gadget had an affordance for accidents with less than perfect RULE adherence - you are silly not to pay attention to it.