While you're correct about the three phases, but this kind of assumes that the reason to buy a MBP or any computer is to win some sort of race to be the fastest, strongest, or best overall. If and when I buy a computer, it is because it allows me to use the available software to do something, such as edit photos, surf the web, watch movies, create websites, track finances, etc.
While everyone wants to get the best value for thier money, what is more important, is will your purchased computer allow you to do the thing you want to accomplish with the computer you purchased.
The one area that may be an exception with today's technology is in gaming. I am not a gamer, but I understand that game software is very demanding and keeps getting more so with every generation of new 3D game variant. If you are buying your computer for gaming, good luck keeping up with the cycle of software leapfrogging computer capabilities. I suggest you invest in an Xbox or Playstation since they are much cheaper to keep upgrading than computers.
However if you are editing photos or blogging or watching movies or shopping on Amazon, I think any computer you buy today will meet your needs for the forseeable future. I have heard the average notebook lifespan is 4-5 years and for table tops about 5-6 years.
The one area where my MBP might be weakest is in the hard drive capacity, but with the excellent external hd's available, this is not a reason to wait. It also is not a good idea to store everything in your computer in case of failure anyway.
I say, if you need a computer, pick the size and speed available that suits your needs and buy it without looking back. If you are buying a computer to play Halo 3, Xbox might be a better way to go.
Best of luck to all in your choice.