I guess I don't really have a problem with the concept in theory but the industry tends to run things like this into the fucking ground. See MP tacked onto SP games, DLC minutiae everywhere, etc. It's no surprise to see these little things popping up everywhere now. At this point just sell me fucking stock fellas!
Borderlands had 1 really strong content addon, 2 that were ok and one that was godawful. I guess at $20-30 that would be an OK value depending on how much actual enjoyment you get out of it. The game itself is supposed to be very lengthy so either way I'll get my fill.