Question:

[THEFERMANATOR]

The main reason that I can think of is that rubber impellers are tolerant of MANY different types of operating enviroments. A metal impeller like found in an automotive water pump does not tolerate sand and grit very well going through whereas a rubber impeller riding inside of a stainless housing could care less. Also rubber impellers are self priming by design, so you don't have to worry about it sucking air and losing prime, and then stopping pumping. Also rubber impellers are flexible and lay over at RPM's which allows them to get to a certain pressure, and just sort of self limit the pressure output without the need for a popoff valve. There are MANY good reasons to run a rubber impeller over a metal impeller IMHO, and the biggest one to me is it has proven itself to work reliably IF maintained.