Yes, except that teenagers don't know who they are yet. They have to experiment, trying out various identities and behaviors until they figure out what fits them and what doesn't. So defining themselves in terms of others, whether by similarity or by difference, is to some degree inevitable.
They do need to be encouraged to grow out of it, though, so your point is very valid.
Looking back on my highschool years and previous, my parents tried to encourage me to grow out of all sorts of phases.
They'd barrage me with empty platitudes like "just be yourself" when my primary frustration in life was that I didn't know who I was, or that being myself so far clearly led to nothing but social dejection. "Just be yourself" became an epithet that mocked me all day from the walls of halls and classrooms, often found block-lettered across posters of celebrities and eagles soaring over canyons.
It did nothing but make me feel like my parents were disconnected from my struggles, unhelpful in the same way "just go up and ask her out" fails to encapsulate dating. Convincing me that it really was that easy, sound advice for everyone in the world but me.
You grow out of phases by coming to terms with them yourself. It's how you evolve.
Well, and I would say it may not even be a bad thing for a certain period. Most of childhood development is imitation. We learn communication, behavior, etc, from our parents when very young. The same imitation exists up until some point. However at some point, it becomes unhealthy.
They do need to be encouraged to grow out of it, though, so your point is very valid.