I keg because I'm too lazy to bottle. Bottling takes way too much time and effort. I can keg a beer, including sanitizing time (walk away and let the keg soak for 10 minutes), in less than 15-20 minutes (rinsing keg - 1-2 minutes, siphoning beer into keg - a couple of minutes). Washing 50+ bottles, rinsing them, filling them, and capping them takes way more time. I don't clean the lines and taps between beers - the new beer flushes the old out, never had a problem with the old beer affecting new beer flavor (although I do change hoses and rubber when going from root beer to beer). My fridge has 5 taps so I keep a good variety on hand all the time.
Having said all of that, some of the best brewing times I've ever had was bottling with several friends helping.