Marking is a useful tool and it doesn't hinder anything at all. If, as a tank, you stop marking and DPS attack the other mob and draw aggro, then you are to blame, not the DPS. Being on the same mob as the tank usually works, but sometimes that tank has to switch to draw aggro on another mob. Marks keep people focused on one mob so the tank can switch without having people switch with him.
As a healer, having aggro issues is a problem. The marking helps to minimize aggro problems.
