There was a theory years ago that you needed to move the beer to a secondary for a couple of reasons:
1. Lots of people were fermenting in LDPE plastic buckets. In theory these buckets were permeable to oxygen. However, in practice, the bucket wall was too thick and the bucket was under a slightly positive pressure from the fermentation CO2. Oxygen had a hard time getting in.
2. There was another theory that letting beer sit on trub for more than a few days caused off flavors. We now know that the trub has some nice nutrients for the yeast and after a few days, gets covered up with a layer of yeast slurry.
There are some good reasons to move to a secondary. Lagers and high gravity ales benefit from some extended quiet time. And sometimes you need to free up some space in the primary.
I do a modified secondary/conditioning step in my keg. After two weeks in primary, it goes to the keg for natural carbonation and aging. Two weeks for lighter beers, longer for big beers. Another benefit of this method is that when the yeast become active from the priming sugar, they are excellent oxygen scavengers.
Anyway, I think I have just beat a dead horse a bit more.