IMHO, there is a major difference between taking what is helpful and relevant for a specific horse at a specific time and causing confusion. As JDI said, you can borrow specific exercises, techniques, and games, from many different trainers and have a coherent program that gets good results. For example, you could start a ride by doing a little Lunging for Respect, Friendly Game, and Grooming and Showmanship to warm up on the ground, mount up and do some lateral flexion, walk off on a loose rein, move into trot and pick up a feel on the reins, ride some 20 meter circles, change rein, do leg yields from quarterline to rail, pick up a canter, canter a figure 8, drop stirrups, transition to walk, slowly put slack in the reins and knot them out of the way over the horse's neck and ride simple figures without them to cool down... on and on. That might be similar to one of my afternoon workouts with Scout, borrowing several "name brand" exercises, plus a few figures that have been around as long as people wanted their horses to "dance."
I have a darn strong suspicion that this is how many of the Parelli's own Games and techniques were developed. Pat certainly borrowed many ideas from his mentors, and put them together, mixed, obviously, with his own ideas and spin on the base philosophy, into a system that delivers results if applied correctly (this caveat applies to every
training system out there, NH, BNNH, or the guy with the roundpen down the street who saddle breaks colts for neighbors.) Every trainer stands on the shoulders of the one(s) who taught him/her.
The problem comes when you try to accomplish the same task in 2 different, potentially contradictory ways. For example, my horses learn early how to back up off of lead rope pressure backwards, like a G&S horse. Eventually, they get really sharp and will back up with my shoulder as I walk back. However, the Yo-yo Game seems to completely mystify them. Because they associate backing up with a certain feeling, and with keeping their throatlatch beside my shoulder, they don't try to back up to release the pressure, at least not before my wiggle arm is wiggled out.