Running full nodes don't have to expensive, but use Raspberry PI won't be practical in few years.
You don't have to use Raspberry's. It just shows us how cheap it can be.
I recently tried to run my UASF node and succeeded it but I needed to set up an external battery for my pi to protect it against power cuts/database corruptions and that was a lot of work so I let my 5 year old netbook handle the job.
Have you ever heard about BCM = business continuity management ?
You see, no one will ever route a Billion$ Business through some waky shaky nodes. Forget it..