Sometimes i come to the site to ask 2 or 3 question quickly, maybe edit a post or two, etc. Its not uncommon for a captcha to appear for me. in fact it often does (typically when i leave and need to finish a post and i take to long).
I like to request adding logic to not captcha me if i ask 2 quick question but to allow me 2 or 3 quick questions in the span of a 2 mins. Then display a captcha if i am trying to post an unreasonably much in the span of a few mins. I think 3 per 3 mins is fair and maybe 5 question every 15 mins should keep most spam away.
-back button- i just had a captcha come up now! and i didnt take long and i think i only posted 3 answers (superuser, on a one feature per post thread) in the last 15 mins!