Why would I get a new one?
My MBP will be 6 years old this summer, that is forever in computer years. It still runs well, and I never got this kind of usable life out of my Windows PCs, but it is time for a change.
Your choice may be very different; I choose not to keep equipment into the final trimester of life. My kids are out of the house, I'm getting ready to retire, and (other than me) don't need old relics around the house.
On a practical note, my current machine runs Mtn Lion OS X 10.8 very well, but somewhere along the way with OS X 10.9 or 10.10 it probably won't meet the hardware specs and I won't be able to upgrade.
Back to your case. I assume budget is a primary consideration from your comments. You would probably get several good years out of any newish (even a 2007 model) Mac, but if money allows, in general I believe buying the newest and mostest is generally best in the long run.
Others may disagree; that is their prerogative.