Rhythm Heaven is indeed awesome.
Reasons Band Hero should not exist, or at least not the way Activision wants it to exist:
1. Activision already has one successful franchise that uses the exact same specialized peripherals (i.e. Guitar Hero: World Tour and its full set of band peripherals, the same equipment being used for Band Hero). Releasing another music game with gameplay absolutely identical to their current hit series but a different title is nothing more than a blatant cash-grab.
2. Every bit of info Activision has released about the game says it's Guitar Hero 5, except with a pop-rock playlist (with heavy emphasis on "pop") rather than a hard-rock playlist (with, in many cases, heavy emphasis on "hard") and a different name, which leads to my third point...
3. They could release this as Guitar Hero: Pop Rock or something similar and have it sell just fine. Its current name serves no purposes aside from using the copyright they already registered and confusing the non-gamers who buy these things for other people into thinking "hmm, this must be what they meant by Rock Band" and, instead of buying their grandchildren or whatever the Rock Band game they all said they wanted, buying them Guitar Hero But Don't Tell Anyone Edition, Also It's Not Even The Same Kind Of Music The Normal Guitar Hero Games Have. Anyone else remember an MS Paint comic from a few years back that had a dad buying a PS3 for his kid at launch and thinking he was so clever for buying the most expensive console, only to find that it's the opposite of what the kid wanted? I don't doubt that with a name that uses part of the name of another big franchise, this kind of situation will be rather common when Band Hero hits. (Side note: I have a feeling this kind of situation is what led to the only full-price sales of Rock Revolution.)