Having a hardware device reduces the number of things you have to worry about not being under your control. For example, what networking are you going to use?

Having a hardware device reduces the number of things you have to worry about not being under your control. For example, what networking are you going to use? Arista? Great. What's their latest supported APIs for Quantum? Well, they just changed (as in, last I heard they had a pull request open to OpenStack Quantum for support). OK. So Arista's API is now stable, great. Do they actually support what you want to do with your end devices? What are you going to install Glance onto? Is the hardware going to be good enough? SSD? How fat will the pipe be into the switch? Dual homed? Quad? Is that going to fast enough if you want to spin up 100 VMs at once? etc. etc.