[Disk_Contented]

There's nothing simplistic about it.
Your programs actions will be on a servers logs somewhere. Along with the users IP and anything else that can be grabbed. Invisible it is not.

"Oh, bandwidth *is* a consideration".
Nope. mail bots cost nothing. I can spam all day long and It costs me no more than my usual subscription to my ISP. I can use other servers as spam relays. It would add nothing to my costs. Bear in mind that it is third parties that do the spamming.

"It won't be hackable, you can't hack something that does no more than fetch files from port 80 without executing them. Watch out for the length of any given link (buffer overflow) and you're fine."
Really? Don't be so sure of yourself.
Sounds like a challenge? Perhaps a handy trojan pops up to remove your app and take it's place?

"most users aren't bright enough for sufficient would-be victims to adopt it"
You alienate every average joe on the net with that comment.

Given the choice, most people would rather not go near the stuff in the first place.
Why download the stuff yourself and use your own bandwidth that you pay for and may be metered, when you can block it full stop. The net result is the same? Nobody gets a paid click through.

Good luck, but you haven't thought it through properly.