If you fly only VFR, why would you ever consider replacing the dry vacuum pump with an expensive wet pump system?

I would say reliability if a wet pump is more reliable than a dry. I know that the dry pumps are really finicky about stuff damaging them. As a rule, wet pumps last longer than dry pumps.

Usually if you pay more its a better system.

Some websites to help.

http://www.sacskyranch.com/rapco.htm

http://www.sacskyranch.com/faqgarwin.htm