I change my ski every year. First "modern" ski was JD's 9300. Then got a white top Nomad and have upgraded every year to the latest and greatest D3 stick. Every year, after the first set in the new ski, I feel at home and have discovered better things in the new one. Only non D3 ski I have tried is the Warp. Very different ski, but got to my limit pass from the dock the first time. Goes against my non-fragile/easy-to-replace/fitted-with-factory-installed-inserts rule though.
I am lucky on the cost side, as I do not lose money on each change. However, even if it was not the case, I would spend the US$ 500-600 every year to have a fresh stick. As Scot mentions, we use broken down bodies, and an important part of the breaking process is caused by skiing. If we are crapping our most valuable physical asset for that extra buoys, and spending A LOT OF CASH in the process, to me it does not make any sense to save those US$ 500...