The main reason to do captchas nowadays is to keep the door closed. Of course they do not deter more expert-ish people, but opportunity "hackers" aka script kiddies. If you're not using captchas you're just inviting them to run a script on your site. Big sites use captchas, there's no reason for the fediverse to not use captchas. We don't need to be "special" in regards to security.