"This is done to bring these songs up to date with enhanced Guitar Hero 5 features like band moments, expert+ drumming (new for World Tour songs) and the new vocal star power. The game reacts to these songs as though they were DLC and are available in the same play modes," Activision explained.

The Wii version will allow you to download on a track-by-track basis meaning you can leave out the songs that suck *coughcough*whativedone*coughcough*. OK, it's not that bad, but it's too easy, so therefore it sucks in a gameplay sense.

Source