Earlier quoted context omitted.
They can't be used for proof of work. Hashing for say, Bitcoin, works because you're finding a hash that has certain properties (e.g., having a certain number of leading zeroes). Anyone can take the input (to the hash function) you've found and trivially test that it does indeed produce a hash with those properties. Finding the hash is hard, but testing the hash is easy. Doing SETI/Folding work doesn't work like that…
Take SETI for example. If you find an earth like planetoid in a very precise region of sky which takes a long time to sift through pictures of the sky to find then you can just give the coordinates of that point for other nodes to verify. This could be the PoW algorithm.
That's problematic, though. You have a fixed computational difficulty (difficulty can't be scaled up or down).