You wouldn't be able to mine with just a standard PC anyway, you've have to have a row of the highest end GPUs you could find with 4 power supplies..for just 1 PC. They really screwed the GPU market up for a while, now they have their own ASIC mining hardware. Its kinda dead for the everyday person just because of the people that will fill up a server room full of hardware and do nothing else but that.......and those kinds of operations are the only ones left.
Like engine said.... let people that need access use them. Free power...i'd setup a LAN gaming space too!
These days I'm not sure I'd to do the "cafe" thing again (did it 1999-2002) because users these days can be anybody, and some are not so nice (or legal). You don't want to do it FOR kids, either, unless you lock the system(s) down to the point of being useless for liability reasons.
HOWEVER, if you give those systems (properly cleaned, etc) TO a school, charity, or service organization you can generally write off the cost against taxes and let them worry about it.... and save your electricity and bandwidth, too.
^ from my understanding if you have internet setup under a business, and you are allowing public access, you aren't responsible for what goes on.
drop by your local starbucks, torrent 100 movies, starbucks isn't getting in trouble. Just an example. That doesn't mean I wouldn't treat them like rouge clients and restrict access... even have logins that track.. heck even just giving them a login and "telling" them all surfing is tracked to their login would probably prevent them from trying anything shady. By no means would I just say walking and do what you want.
for the easy and quick security can even use some heavily modified host files
You need them? Okay, (in general terms) what are you doing with them that leaves so much time free/available? Depending on speed or capacity I might use them as a rendering module for CAD or CGI, backup systems, a net work array, gateway(s), accounting, database, or (wait for it) a budding google like personal search engine. :)
But if I were doing any of those things I'd never let anyone outside the office get their mitts on them!