Not that any clarification was ever needed, but DLC fo Rock Band on the 360 or PS3 is 100% optional. Considering users may pick individual tracks at will, it's no surprise they are among the best selling items in Marketplace and on the PSN Store.
None of your favorites songs on catalog yet? No need to buy. Choice being the key word.
It's not a case of "nickel and diming" unless you feel the game should have been released with an open catalog of what? All songs ever published?
Constant influx of new tracks is what gives games like GH and RB extra longevity, something that is lacking with the Wii, with no change planned. 5 pre selected bonus tracks are not even a consolation prize by comparison.
Regardless, it should still sell extremely well for the Wii, but for those with multiple systems, I see absolutely no reason to choose the Wii version over the PS3 or 360 version because of this.