That would be good, except that a 4E rogue doesn't need to put effort into getting it off. That was true when the game was first released, but after all the splatbook power creep, the expectation for a rogue is to easily use sneak attack on 100% of his attacks.
He actually has two at-will powers that do precisely that, yes.But he doesn't just say "I want to deal +2d6 Sneak Attack to this enemy, make it so!".
And if the ranger always does his bonus damage, and the rogue always does his bonus damage, then the game no longer has a meaningful distinction between the ranger and rogue classes.