Rock Band: You never have to convince yourself that Ruby Tuesday is a good restaurant or that five days is an acceptable amount of time to go without a shower. Your spouse, significant other, dogs, and kids all get to come along without making anyone mad. One major drawback is that you haven't actually left your house, nor has anyone actually attended your shows. You do save on gas.
Real band: You see the world, you see your friends, you try different foods, and you meet new people. Getting out on the road is the way you discover that you're not alone.
I suppose it's pointless to try to break it down in this way, into a dualistic Rock Band vs. real band. Not even the creators of Rock Band could possibly believe that playing the game is tantamount to making your own music. There is, however, a sad similarity between Rock Band and some actual bands, and that is the attempt at realness. With so much of music blurring the lines between ersatz and authenticity, at least the Rock Band game is a tribute to rock, rather than an affront. In the realm of fakery, I would choose Rock Band over American Idol or over any of the other flimsy truths masquerading as music. With Rock Band, you can play along to Black Sabbath or Nirvana and possibly find new ways of appreciating their artistry by being allowed to perform parallel to it. Rock Band puts you inside the guts of a song.
These days, it might be easier to exalt the fake than to try to make sense of the genuine. But maybe by pretending to be in a band, there will be those who'll find the nerve to go beyond the game, and to take the brave leaps required to create something real.