However, you should never need CAPTCHA to login (except possibly anonymously; Fossil requires a CAPTCHA to login anonymously), or to do stuff while logged in. You should not require CAPTCHA to read public information either, or to download (since you may wish to use external download management; for example, I prefer to use curl to download files rather than using the web browser, and it seems that I may not be the only one).
Of course manually entered spam will still get through even if you do use CAPTCHA.