Because ROLE playing. When you play a cop, you should play and do things that make sense to a cop, that means investigating, that means being an enemy to the underground (or friends if you are corrupt). If you play a rocker, you rebel against corporations and tyranny, if your a corp, its all about making money, crushing rival corps and sucking every last cent from the consumer masses for your checkbook, a netrunner should be living on the net, hacking databases, causing mischief, becoming a legend to other hackers..etc.
Blank slate? so your going to investigate for a cop, play a guitar, then help a corp crush a rival organization, then put on some netgear and do cyberspace to impress hackers? To me that is just very lazy game design that bethesda is infamous for. I am not saying there should not be any overlap, obviously that would happen, but each role should feel unique and certain missions that are only availible to certain roles should reinforce the role you have. At least in my mind.
Some games a blank slate is good, in this case it is bad because we have roles in the books which the game is supposed to be made from. Like I said, vampire the masquerade did roles pretty well. I would like to see something in that vein if possible.