What replayability? I will NEVER ever play a pre-gen character, even if I replay the game a 100 times. For a large number of people this is exactly the kind of thing that tends to be very black and white. Either you accept pre-gen characters or you don't. So in reality it takes away replayability for me. After I have played the game one time with a custom character, I will have experienced all the (very limited) character content available to custom characters (minor differences for race/class not withstanding). The only way I would get to expose myself to any new/different content would be to try one of the pre-gen characters - which I flat-out will not do.
Or you could try different race/classes/background/subrace/subclass or choices in the game to get a radically different play experience.
