Some people, me included, enjoy the ride. I could care less about being max level or doing anything "endgame" I enjoy the journey and story of each and every class.
I have been playing since release, with a few weeks break. My highest is a level 45 and I am in no hurry to get to 50 to end up grinding dailies, flashpoints and ops I have no interest in, and I could care less about pvp (I will and do play fps's if i want pvp).
Every MMO I have ever played I have played the same way, and every one of them I have played for at minimum 1 year. I guess thats not a "valued" customer, but the "high end" gamers who will quit because of a class change/nerf and so on are the "valued" customers. THATS the point.
So what is the benefit of a level 50? That I no longer play that level 50 for lack of storyline content outside of flashpoints/ops?