I have only experimented a few times with primary-only brewing. Right now, I wouldn't hesitate to do an extended primary (probably 3 weeks) with lighter gravity beers, hefeweizens, etc., but for anything with an OG above 1.055 or so, I'd probably secondary it.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.