ok, from my experience like you said you can rank a site,post or any url with pure on-site SEO an backlinks so I think using categories or posts as if they were micro niches websites would work,right?
Yes, it works, but be aware, that if Gugl thinks you did something wrong, you may get a big penalty for your whole domain, incase you do big spamming at your site pages.
Otherwise, as far as I now, Gugl is more then ever now penalizing only URLs, not the whole domain, but they also give -30 penalties and such, so that you don't really realise, that you have got an penalty.
In the other site, you use micro-niches, as you don't always have one site which fits with all the low competition keywords you may find on your way. So here you use the power of exact domain names, and I don't see Gugl to change things here for now.
All in all, I prefer to use pages for keywords, if they fit to a site I have, so you gain juice and relevancy from all the other backlinks you already have pointing to your whole domain.
Just don't spent to much time for crap keywords. Do one, and move one. Of course, depends at which kind of keywords.