It's absolutely a myth that you need roundwounds to play rock music. However, it is true that flatwounds (especially after they're broken in) have a lot less bite to them, which leads many players to write them off entirely.
You can compensate for the natural darkness of flatwounds by boosting the treble on a preamp, playing with a pick, using a bridge pickup, or by changing them as soon as they start to get dull (Steve Harris does this, but it's probably not feasible for most of us who aren't sponsored by a string manufacturer and aren't big time rock stars).
I find the "bite" button on my Sansamp does a good job getting me the tone I want out of my Chromes or Labellas. They still don't sound like roundwounds, but then, I don't want them to.