I saw a few of those clusters at a Pi Jam, but not sure what they were used for. I guess a cluster for around £100 is just too hard to resist.
I've got several Pis at home. Only one in active use for now. Got 2 Zeroes, so I could make a cluster Think I need a 3B+ anyway.
FrankenPi are original Pi's I think? Given I wanted to buy the new Pi 3 B+ any way I thought I might as well get the rest. The Zero's are like £9.00 each, the 16GB SD cards I think are a tenner each the Hat is £28.00 so in total something like £150.00 which is actually peanuts if you scaled that up to like a Blade server for example.