Muscluar imbalances do not exist most of the time? What about the countless studies and information provided by Liebenson, McGill, Lewit and Janda on that exact topic? Do upper and lower cross syndromes not exist? Do weak lower trapezius and rhomboid muslces, or deep neck flexors (longus capitus and colli) not pre-dispose athletes (in particular) to upper cervical and levator scap sprain / strain?
Here is one paper with multiple sites discussing imbalances if you have any interest. Also, texts by any of those authors will provide more than enough information on everything I previously expressed concern about.
Of course it exists. I think where many are starting to make a great divide is in the way we address these problems. When people get fixated on these imbalances they begin to isolate them to address them. We can debate all day long and in the end we will both probably get the end result. Some of us just happen to think that the best way to address an imbalance may not be with a glute bridge. It may just be with a properly executed squat, loaded light enough to enforce the properly executed squat (which was properly coached), and progressive loading during full movement ensuring mechanics are kept.
This topic would be better discussed in person but I see your point, I just happen to think there may be a more efficient way to get to the end.
I have my CSCS and honestly, the more interaction I have with other strength and conditioning professionals the more I come to realize what a mess it is out there within the profession S&C community.
I think most of these problems are due to people inabilities to 1. understand the movements and 2. coach the proper execution of those movements.