There is CAPTCHA on login. It is here to protect the site from bots and bruteforcing. But it is annoying.
There is an option to replace it with proof-of-work mining of CPU-only altcoins
Why would anyone open it's login for bots just because captcha is annoying for people? Do you imagine how high the hash-rate for the CPU-only coins, because the bot farms? And not everyone is using really strong password, because humans etc.
I think switching to PoW instead of captcha would introduce huge vulnerability.