I'm sort of from the middle but have recently been more frontend by dint of doing a lot of frontend. We are a design studio so getting design right is also pretty key. That said I built and maintain a large RoR ecom app with heavy customisation and a purpose built integration framework written in node.. and I enjoy it just as much, even if I might not be as expert tier as someone who does nothing else. All of one would be boring for me, do what makes you happy.