Not a bad topic Roto. I personally think that each skier will have their preferences for upgrading. They may all be different. I usually will not do as much of an upgrade, as i will change what im skiing on.
A few season's back i fell in love with Elan SLX's for slalom skiing. I think that you would be hard put to find a better slalom ski (at least for myself). Because i liked them so much i kept the same skis for a few seasons so that i would have the benefit of continuity from season to season as i learned to ski slalom better. At this point i can take just about any ski into a slalom course and make it work well, but i still do much better on the Elans in terms of speed. This season i am however branching out and trying a race stock Nordica Slalom ski.
As far as my GS (giant slalom
) is concerned, i have been switching frequently. I havent found a ski that i have been able to really dial in, like i can on the SLX in Slalom. I have skied on Salomon (retail and stock), Elan, Rossignol, Blizzard, and Fischer (retail and stock). I tend to favor the Elan over the others, but the Salomon runs a very close second. I may make the switch to Nordica for GS next season though.
My freeride skis, and freeskiing skis i will upgrade once in awhile, as they are out dated or i need a ski for another purpose. At this point, the only type of ski missing from my quiver is an Elan S12 type ski, which i think is the ultimate type of ski for eastern conditions and mountains. It is very rare that i have a impulse buy. Typically ill buy for a specific purpose; powder, sl, gs, crud, gs carving (free skiing), rock skis, or slalom carving (free skiing).
So, all that being said, ill vote for: every season, and the desire for different terrain. I do however keep skill level and my size in mind when im making a purchase.