Logon forms are another whole issue. "Lock out the account" is just a DoS vector. People are quick to talk about systems that can defeat a captcha but if the brute force goes from 50 passwords/sec to one password/10 sec it's mission accomplished.
I worked on Google's system for solving this. It's a pretty sophisticated analysis that decides whether to demand CAPTCHAs or not based on a lot of factors. The CAPTCHA was needed (at that time) because it's the only way to slow down the attacker without bothering the real user, who won't be shown one.
bullshit