It's too bad that OpenMosix withered on the vine. Back when it was a thing, I found it really handy to fire up clusterKNOPPIX on a bunch of idle PCs for large compile jobs.
I guess with today's multi-core CPUs, it wasn't thought to be worthwhile, but it could still come in handy for resource-constrained systems like the Pi.
I share your sentiment regarding Mosix. I don't think however, that clustering SBCs is of any value beyond the learning experience. Even a moderately powerful laptop is much faster than a *Pi, e.g. a 2013 Chromebook Pixel is about 16 times faster than a Banana Pi, when measured using `openssl speed`.
It depends when you're trying to do with them, but the network speed of the Pi is definitely a bottleneck for me. If it weren't for that, the throughput could definitely be used for some cool stuff even with modest CPU/RAM.
I guess with today's multi-core CPUs, it wasn't thought to be worthwhile, but it could still come in handy for resource-constrained systems like the Pi.