The pommels main purpose should be to hold the hilt onto the tang and your to stop your hand sliding of the end of the hilt. A pommel that is heavy is an
indication the sword may not be balanced right. (See buying a sword for information on the swords point of balance).
Using the pommel to knock on heavy things like
doors must also be viewed with question. The last thing you want to do is damage the pommel, since in a good sword, it is the only thing holding the hilt to the tang. It would be embarssing for your hand to slide of the hilt, but even more so if the hilt and your hand slide off the blade!