It only needs to be wasteful for whoever is doing the computations. It can be useful for other people. Especially if it's not worth a lot of money.
For example, if it were solving an incredibly complex mathematical or scientific problem that benefits all of mankind - then the incentive is equal for everyone.
If you come up with something useful, but noone is willing to buy it at any price, what's your basis for claiming that it's useful?
It also happens that both these conditions (intellectual product + must be published freely) are the typical approach of PoW blockchains.