I understand your frustration. Logic is very solid.
I can think of the following positive aspects of these projects:
- Some people may enjoy finding primes or amicable numbers, thus it's not more wasteful than playing computer games for fun.
- These projects might help to stabilize gridcoin network. I see there are quite often situations when projects fail to deliver new WU. For this kind of math problems it seems very easy to generate any number of new WU.