What I meant was Additional security, not optional.
Think of it as another way to verify the transactions/block/chain on top of the existing one. (using different proof-of-work)
So far it seems prime number search is the best for general CPU's people have at home.
From the benchmarks I've found the Core i7 is actually beating GPU's easily.
Folding proteins etc would also be okay. At present GPU's have about 10x advantage on CPU's.
This is way better than the approx 300x advantage they have with SHA256.
difficulty could be set by the size/length of the factor found to be part of the test number.