I'm with Tom Sawyer on this one.
I think it's upstanding of Bill to do what he can to develop a toilet that's practical for a place like Africa where they don't have running water and need the fertilizer for their fields. But, that's not the only reason...
It's solving practical problems like that that motivated NASA to build the first cordless tools for use in outer space. And look how far that technology has gone.
With nearly half of all new discoveries being made completely by accident, you never know where a great idea will come from. Via-gra started off as a drug for heart disease, but the people using it noticed it had an amazing side effect. Formica was originally intended to replace the mica shims in electric motors until a waitress in a cafe suggested it would make a good material for restaurant counters and table tops. Teflon was discovered when some lab scientists noticed that tanks of flouride gas weighed the same as when they were received from the supplier, but didn't have any pressure in them. They cut the tanks open and found a previously unknown material inside. At the time, no one would have guessed that plumbers would make use of that material for sealing threaded joints or that it would prevent eggs from sticking to frying pans.
I applaud Bill Gates for spending money on researching a new toilet since there's no telling where the knowledge gained from that venture will eventually be used. As long as you spend the money doing the research, you WILL find answers. It's just that you never know what problems those answers will solve. The results of this venture could be anything from "nothing at all" to a whole new technology involving a new kind of bacteria, to just a toilet that's practical to use in Africa, to a new way of tuning pianos. There's just no predicting that kind of stuff. But, if it's an answer, someone somewhere will use it for something, and that's how we make progress.