Yeah, it's basically just transport encryption.

With basic SMS, anyone with $100 or so of equipment (like, police, or even a bored and resourceful child) can fake a tower and force-downgrade your encryption to read your messages. Your cell provider also sees your messages.

At least with iMessage or whatnot, you can rest assured that only Apple can open your messages. And, I assume it's not standard to open them (for analytics and marketing) unless there's a warrant.