Whilst it's easy and quite frankly justified to hate the onslaught of [word] Hero games, I think it only got out of hand once they starting pumping out identical titles under different names.
Guitar Hero II and III had obvious improvements over their predecessors, which is a good thing. They were releasing new games that actually moved the series forward and developed the game a bit. But once they began grinding out the individual band games (Aerosmith and Metallica) is when they stopped going anywhere. Whilst I was interested in the Metallica game, as a fan of the band, they didn't actually put anything dramatically new in there other than the songs and some characters/items/venues. Something they could have just added as DLC, even it the download pack cost £20 or so I think that people probably would have bought it rather than a separate disc.
Then they started branching out. Band Hero and DJ Hero both look and sound pretty dull, but they will sell because people want achievements and will undoubtedly buy/rent the game which further encourages the developers/publishers to give their game pump another squeeze.
I've never played Rock Band because of my deep down disliking of EA. Ironically because they release countless hollow games in the guise of sequels, which Activision have also started doing. But there isn't much difference between the two games other than the fact they have different controllers and names. The only fun that comes from the [word] Hero and Rock Band games is guessing who will release more 'unique' titles in a year.
In a nutshell, there were too many music rhythm games after Guitar Hero World Tour. And there shouldn't be any more unless they come up with some seriously cool new features.
