Bots take a lot of time to build, program, test, etc so the only people who will probably take the time to make a bot are people who are trying to make money.
Spam questions: humans
Flame wars: humans
Advertisements: bots and humans
Captcha does not capture humans. This means that it it will not prevent flame wars and spam questions.
But, I am actually in favor of it. It will be a one-time thing (since it’s on sign up and not sign in) that will prevent ad bots.
Something else:
When you join Fluther, you must provide an email address. Is a confirmation email sent that requires you to confirm your account? I can’t find one in my inbox. I do have some Fluther emails from as early as January 2011, but I may have deleted stuff earlier than that.
If a confirmation email is sent: On most sites there is a period, immediately after signup, when you are logged into the site even though you have not yet verified the email address. This provides the opportunity to someone (aka a bot) to enter a random email address into the registration box, get logged in, and commit spamming without actually creating that email account. Later, the bot will be unable to log back on, but that doesn’t matter anymore.