Activision think they know how to "win" the music game arms race. They think signing big artists to "exclusivity" deals - where they can appear only in Guitar Hero games - will help tip the balance in their favour. They can think what they want, it's rubbish, as all it does is piss the people off who already bought the "other" music game. Plus it makes Activision look really mean. Refreshingly, that's a sentiment shared by Harmonix's Eric Brosious, who told IGN:
We prefer not to sign exclusive deals with artists because while it seems like the competitive "business" thing to do, in the long run, it's really not good for anyone. We think we should be working to get more music out to more people.
Our hat is tipped to you, sir. Rock Band 2: Behind the Music [IGN]