The lens is too big and havy as a walkaround lens compared to the EF 100-400 IS. The solution with the build in TC is not amazing. For the mentioned price I prefer the normal big whites.
this lens is not meant to be a walk around lens.. just like the 400 f/2.8 and 500 and 600 f/4's.. you don't see people walking around town with those unless they have a purpose to.. its meant for sport, wild life, and journalism when you need flexibility and be the fastest possible lens you can get. hence the price tag.
the 100-400 is more of just a general purpose tele that does everything which is why it is smaller and lighter
as far as the TC goes.. its for connivence.. so you don't have to take your body off put the TC on and put your body back on.. Just move the lever and you're ready to go! just like TV lenses (which is what that stems from) it just makes things easier on the fly so you can get that shot more easily