I recently did some research on PPC searches containing different numbers of words (i.e. short, medium and long-tail searches), and came to similar conclusions to yourself: that long-tails often rank better because they have less competition, and that since people making long-tail searches are often further along in the buying cycle and are therefore more motivated to buy, conversion rates are often much higher.

http://www.alanmitchell.com.au/techniques/benefits-of-long-tail-keywords/

I also found that since people making long-tail searches are typically more specific in their needs, long-tails present a great opportunity to write highly targeted and tailored ads and achieve a strong CTR. Although the research was carried out with more of a PPC focus, the general principles still apply for SEO.