What if someone else with malicious intents increases difficulty for your nickname, intentionally making wrong attempts ? Ooops, and in order to login you need to do a couple of giga-hashes. Or, if you wish, we can do it for you, for a small amount of ether... See how it works?
Again: users should be able to choose captcha as alternative to POW. They could do this in such cases.
That doesn't fix the issue though. They're then still going to have to use the captcha?
What if, at some point, the user who was using 50000 IP addresses returns to try to bruteforce the forum again and attack everyone's user accounts and makes it extremely difficult for everyone to log-in without the captcha. Also, it'd use a lot of unnecessary CPU power to try to mine it - especially if they don't know how long they'd have to wait first.
Attacking thousands users and making it impossible for all of them to log in without captcha? This will give much coins to the forum!
Users will just log in with captcha and reset difficulty. They should be able to see what were the failed password attempts and laugh at how far they were from their real passwords (increasing difficulty will not allow many attempts, remember FLM).
We wouldn't want a rush of environmentalists that start to complain about how this forum is contributing to the decline of the environment now would we?
Aren't those environmentalists already okay with ASICs and GPU farms consuming electricity more than a middle-sized state? CPU-mined altcoins will save us from this.
Again: users should be able to choose captcha as alternative to POW. They could do this in such cases.
Defeats the point then. They'll just continue using the very system they are using now and allowing people to login with their CPU would be a nothing less than a gimmick.
Spending CPU-time mining coins for the forum... is it not a charity? And what is the purpose? Just to make users use captcha again?