I know people with Macbooks that are 8+ years old that are still chugging along.
It all depends on how you treat your machine and what you do with it. If it does everything that you want it to do and you are happy with the performance, thre is no 'need' to upgrade.
If you are uphappy with the performance of your machine, or the size and weight of your machine and want something faster/smaller/lighter/sexier, then go for an Air and either sell your old Macbook, repurpose it (to say a media server or iTunes server), or pass it along to a family member