I find this happens if the tanks don't trust or feel comfortable with the healers. I have seen both sides where I'm the tank and the healer is not keeping me alive - i will use clemency to get me back up every time. (this is primarily for 4 man content in PUGs) clemency is there for a reason and if the healer is lagging, or for whatever reason letting my hp go below 20%, I'm casting. As healer, I have seen this and if the pld wants to constantly use clemency, i give him a regen and i dps.

At the end of the day, if casting clemency prevents the party from wiping i'll cast it all day long. I do see the OP's and others opinions and thought i'd offer mine.