ignoring all the previously mentioned issues:
if this were to happen, the core rock band fans would gain a grand total of... what, 300-400 songs? while the core guitar hero fans would gain somewhere close to or above 2000 songs? who would win that deal... I can't see HMX ever going for it,
although i own over 1000 DLC, guitar hero has 10-15 songs available that i want more then about 900 of the songs i have... but that doesn't bother me, since in total i'd only have bought MAYBE 40 songs activision has released as DLC.
the merge idea just wouldn't work