From a quality perspective, secondary isn't necessary. There have been more than a couple of experiments to demonstrate that having your beer sit on your yeast cake and trub for a few weeks isn't going to leave much in the way of off flavors. The main one I can think of off the top of my head is the collaboration between homebrew radio and BYO magazine.
However, more often than not I do transfer to secondary, as I find that kegging from secondary tends to suck up less sediment from my carboy, leaving less sediment in the keg in case it needs to travel, which it often does. Also, I tend to repitch the yeast from primary onto my next batch of beer, and the yeast is less viable after it's been sitting in its own waste (alcohol) for a few weeks, and I also need my only two 6g carboys again for that batch.
Now with that said, transferring to secondary runs the risk of introducing oxygen into your beer leading to off flavors, but also kicking off a secondary fermentation if your wort wasn't sufficiently oxygenated initially, and depending on the gravity and style of beer, this might be a good thing. But I purge the 5g carboys that I transfer into with some co2 to minimize oxygenation and seems to work well.