I'm pretty sure it is essential! The rpi's I/O use 3.3v, the Bitfury chips will not like that (i.e. die horribly), as they're meant for 1.8v I/O...

I would also be very wary of power-sequencing issues if you are not powering your rpi from your rig - I had a try at using opto-isolators to separate the rpi and miner power domains, but my optos were way too slow for the SPI. Now my board supplies the pi with 5v directly.