Computers like a Raspberry Pi, where the OS is on a sdcard, will hugely benefit.

Yup. There's lots of advice about how to reduce cycle count, increase lifetime of sd cards out there. This post has a bunch of ideas, and tmpfs is definitely on the list. https://raspberrypi.stackexchange.com/a/186/32611

Technically it'll have some impact on the number of write cycles your disk goes through, and marginally reduce the level of wear.

Most disks have a lot of write cycles available that you'll be fine anyway, but it's a tiny benefit.