Yes, it does that. It disconnects the poly proxy from the subdiv model when you delete the history. I'm not sure what happens if you delete the history while in standard mode.

Anyway... To keep subdiv performance as good as possible, you should try to create only quad faces in the poly proxy mode.

I personally work in pure polys as far as I can before turning the model in to a subdiv where I only add some finishing touch like creases...