Fame
The most famous plumbers in the world are Mario and Luigi. So unless you're an 8-bit animated adventure artist on the side, you should probably resign yourself to a life of anonymity.
Then again, you never know what you're going to find in those ancient pipes. Maybe the long-lost golden medallion of Azkatar will soon be within your grasp. Once you clean the poop off, it's really going to be worth something.