It isn't really a matter of overbuying, I think that's actually generally wise when purchasing a machine. Get something more than you need that you know will last.
It's the overbuying every two years that I see that makes me chuckle. Most folks don't need NEAR the power to justify a new machine anywhere close to that often, I know lots of pro photographers, designers, and animators that are using machines apparently a lot slower than your average Macbook Pro forum poster.
Yea but how do you know how long it's going to last when you but the machine in the first place? I purchased my MacBook Pro in 2011, I maxed out on specs with 2.2GHz i7, 8GB Ram and 750 HDD. But I didn't know how long it was going to last. Don't get me wrong, I'm glad its still working great. But I still don't know long it will last. My needs are evolving, with conmuting too and from univeirsty I could do with a lighter laptop and that's why I considered the MacBook Air, but that has nothing to do with how long my 2011 MacBook Pro is lasting, because it still works.