O.K.--I'll break this ice on this one:
1. Code Protection
2. Ability to feed specific pages to specific engines
3. Ability to modify pages that "visitors" see without
changing well ranked pages
4. Ability to experiment with meta's, code, keywords, kw density and still serve a regular, attractive page to visitors.
1. It is percieved as something dirty or underhanded.
2. It is easy to abuse, so people will abuse it, by spamming, codejacking, etc. (which leads us back to con #1)
3. There is always the possibility that SE's will aggressively try to detect it and start banning for it.
For the record, I do use cloaking. I would say I use it pretty conservatively. Most of my cloaked pages are nearly identical to the visitor page I just tweak them a little bit for each engine. I have had moderate success with it as far as SEO goes. My main reason for employing it was code protection.
O.K.--now you have a scape goat--Fire away!