Personally, I view any computer as a tool. It's not a showpiece for others to ooh and aah over, it's not an investment vehicle, it's merely a device that I use to accomplish my real goals. Those goals may be photo editing, document writing, spreadsheets, communicating via email/IM/skype, etc.
As such, new technology is only relevant if it will substantially improve my ability to achieve my goals. Lets say a new MBP version offers 30% more CPU processing power and 25% more battery life. (I'm making up numbers here) If I am rarely constrained by the existing model's battery life and if I'm rarely waiting for a CPU intensive task to finish, then the new version really don't give me a real-world benefit. Sort of like buying a larger refrigerator; if my current one is never full of stuff, what benefit would a bigger one give me (all else the same)? None, so there's no value in trading up.
This is why I suggest looking carefully at what you do with your computer and considering thoughtfully what specific areas might exist in which it does not meet your needs. Then when new versions come out you can more easily ascertain whether the new versions would serve you better; and if so, does the value to you exceed the cost of the upgrade? In this situation, upgrading every other or every third generation is often the best strategy.
Some people do value having the latest and greatest just for the sake of being able to feel or say they're at the cutting edge. In those cases, upgrading with every new revision makes sense since they derive value from the object rather than what it does. In these folks case, it makes sense for them to upgrade with each new generation.
