I have one of those tensioners sitting on the bench, soon to be used. Prior to insertion, I will use a saw to lighten it by cutting off all of the tensioner-related parts. No way am I gonna run a steel chain against a $3 plastic piece and expect it to not wear. I should also add that I believe (?) those tensioners were developed by Chrysler for use with a link type chain, not a roller. That answers your question of "if they're so bad why do they last so long in stock engines?"
I put one on my 273 with a roller chain. I wouldn't do it again. Yes, they are designed for standard chains and not double rollers. If I ever need to take the front of the engine apart I will take it out. Not necessary with a good double roller.
