The lowest-spec'ed cameras in each manufacturer's line are updated so often, I see little reason to upgrade each generation, while remaining at the lowest-spec'ed level. Why not move up a level?
Even at the semi-pro level, neither of my mentors believes in necessarily buying new each generation. One of those mentors is my wife; she is wanting her D7000 to last through 2015, and sees no logical reason to get a D7100, even as a second body. (She has a D70s that can act as a back-up body, and has access to her employer's pool of D200 and D300 cameras, if her D7000 fails while she is at work. She is a forensic investigator for the county Medical Examiner.)
In the Nikon line, only the Pro DX category, represented by the D300s, is significantly in need of an upgrade, and it is actually not clear whether Nikon wants to keep that part of their line going, perhaps wanting to steer their customers toward the D7100 or FX.
Canon 5Ds R/7D2/7D/5D/40D/1D2N/M3; Nikon F6/D3s/D700/FM3A/1Dx/Coolpix A. Lens selection undergoing changes; some favorites: Zeiss 2/135 APO Sonnar, Canon 100mm f/2.8L Macro IS and 135L, Nikkor 14-24/2.8G and 24-70/2.8G.