Burpees are definitely fine for warming up if you ease up on the effort, I have no disagreement. It is just my opinion that burpees are a great "whole body" bodyweight exercise and should be utilized to its full worth as a core part of a workout routine rather than a warmup. I somehow see that as almost wasting the movement. Throw burpees into a Tabata interval or add weight to the burpee and now your talking work. But sure....you can use them as a warm up...why not...I have no problem with that. I would still save any serious stretching for the end of the training session. Maybe a little after you develop a little sweat and then the major portion for the end.