What would be the issue? Is it any more likely to go farbot than a more beefy machine (probably less, due to the lack of moving parts). Never mind that the author is using it to backup his email. It would probably run some kind of fetchmail script as a cron job and caching it locally, so that you don't have to keep a full-fledged computer running all the time. And once that's started up, the backup could easily be downloaded, so that you store your data on several machines.

Pretty common setup, people do stuff like this quite often with their routers.

Why not? You can use mdraid on removable devices, which takes care of the single-point-of-failure problem...

considering you can get a fail-over for another 30$, why not?

the pi is a just a tiny computer, the usual rules and best practices apply.

ffs, if you want, you can run a mail server on your _phone_ which has more raw computing power than a pi and similar power consumption.