For performance testing, yes. Absolutely.

My point about threading is simply that if you have a 32-way processor, unless you are constantly running 32+ separate tasks, you're wasting some of that power. However, if some or all of your your less than 32 concurrent tasks are set up for threading, then they will benefit (a little or a lot) whenever there are less than 32 tasks running.

And there are many tasks, like the OPs primes algorithm and the other examples I cited, that do not lend themselves to being multi-tasked through forking, because they need access to shared data.

All the problems with threading lie with the nature of the low-level abstractions for controlling shared memory access. The language that makes that easier, preferably transparent, will clean up in the future.