i'm in the lucky situation that I can afford any mac I want. However I still can't justify to spend money on a maxed out Mac. So I stick with my 2010 MBP for a few years more. I might put in a SSD as soon as I get around to it.
But in essence I only buy what I need (well a small bit above that because i could do with an i5 but I bought an i7). I also keep my notebook for several years because I just don't like to waste money on gadgets without really using them to the fullest. I still use an iPad1 and never upgraded. I used my iPhone 3GS until last December and only upgraded because the 3GS broke.
Gadgets tend to not only cost a lot of money if you upgrade too often to the top model. It also costs a lot of time and effort to upgrade, transfer software, learn the new features, sort through the accessories and adaptors and all that. It's just easier to use what you have until something groundbreaking new is there.