There are a lot of people who hate Rock Band and Guitar Hero, for one reason or another. Phrases like "learn to play a real instrument," "stupid plastic instruments," and "when will this fad be over," are constantly thrown around. Some people even complain about their favorite bands "selling out". I'm here to say that, at least for me, Rock Band is not about playing make believe and pretending to be a real rock star.
The first and most common complaint I've seen, especially on YouTube, is deriding people who are good at the games and telling them to learn to play a real instrument. Many of the people who are really good at the game, like YouTube users guitarherophenom and IBitePrettyHard, actually already do play real instruments, and quite well. Other people, like me, don't care about making music. If I wanted to learn to make real music, I would learn how to play the guitar for real. You'd have to be a complete dumbfuck to think that playing Guitar Hero is anything like playing a real guitar (Nobody does. At least I hope nobody does. People's stupidity has surprised me before, though.).
For me, the appeal of Rock Band is in skill progression. Rock Band and Guitar Hero are the only games I have ever played where I actually got noticeably better over time. I started out playing Guitar Hero III at a friend's house in the summer of 2008. I played on easy and missed about a third of the notes. I kept going over to play the game (he lived across the street from me), and eventually I was able to play on medium, then hard. Then, at the very beginning of this year, I moved away, getting my own copy of Rock Band 2 with a guitar and drums as a late Christmas present. By the end of this summer I had beaten Green Grass and High Tides and Painkiller on expert guitar, things which I wouldn't have stood a whelk's chance in a supernova of doing a year before.
No other game has ever done that for me. Playing something like Call of Duty 4 on veteran difficulty wasn't really challenging to me. Tedious, but not challenging. I had gained nothing in the way of actual skill by the time I had finished it. My aim was no more accurate, my reflexes no quicker than when I had started the game. The only reward was the ability to say that I beat the game on Veteran. Something I probably could have done when I was ten if I had had a strategy guide. Whoop-de-fucking-doo. It just felt like a complete waste of my time.
With Rock Band, it was really easy to notice how much better I got. My ability to process the notes coming down the screen got better and better with practice, as did my fingers' ability to hit the buttons in time. I learned tricks like anchoring my index finger on the green for songs like Painkiller, Panic Attack, and Get Clean.
I've noticed a lot of people talking very favorably about Demon's Souls' difficulty. Specifically, about how it encourages the player to keep going despite the immense difficulty, how it's always fair, how rewarding it is to advance, and how you actually get better at the game by the time you finish it. Every single one of those perfectly describes Rock Band.
My sense of rhythm improved drastically. I listened to music differently, hearing each instrument track as a separate entity. The game introduced me to Metallica, who I had previously thought to be terrible. Now they're one of my favorite bands. I gained new respect for AC/DC after playing through a few of their songs (they're my second favorite band now, after Pink Floyd). In general, I grew to love music even more than I had before, and the music I listen to is of a much wider variety, directly because of music games. Now, I'm even thinking of getting a real drum set and learning how to play for real.
I guess the point of this post is that, if you're put off by RB/GH because you think it's only about make believe, then there's a different way of looking at it and a completely different appeal. Also, if you like difficulty and feeling rewarded, then there is nothing more rewarding in my experience than finally beating a song like Battery or Green Grass and High Tides on expert difficulty. Nothing.