What about dry hopping? Can you do that in primary or is that a qualifier for secondary?
FWIW, I still consider myself somewhat of a novice when it comes to homebrewing, but I've never used a secondary (thanks largely to the great advice everyone has provided here). I'm a huge hophead, and I generally dry hop the hell out of my beers. I've used both pellets and whole cones, no hop bags, and just sprinkle them right into the primary. The results have been great. Unless I start harvesting my yeast, I don't think I'd ever bother with racking to a secondary for dry-hopping.
To quote the late Slim Pickens from the movie Blazing Saddles, "Ditto".
I follow the same protocol. Beer goes straight off the yeast into a keg. If I'm dry hopping, I do it in the primary and/or the keg. Loose in the primary and in nylon bag in the keg, or in an infuser.
Per Denny's advice and Jamil and John's great book on brewing classic styles, I quit racking to secondary years ago. This isn't a set in stone procedure for all beers, but it works for mine.
Additionally when I used to rack to secondary I ruined many beers in the past ( it's been awhile ) by sloppy racking and aeration during racking. I thought the beer tasted bad ( oxidized ) because it sat on the yeast for too long. I then found out it was OK to let it sit for up to a month and realized it must be something else.
I tried skipping it once and the first time I did it my beer was so much better I was amazed. You could say I turned the corner at that point but later learned my racking procedure was the culprit more than letting the beer condition in secondary.