Well, unless you do it for power-consumption reasons. Newer chips do draw less power, at least at idle.
But in terms of performance, unless you play games at hardcore settings, you really don't need a new computer. Even a Core2Quad is plenty of horsepower for basic things.
Granted, I think that having at least a basic quad-core, or a SB/IB dual-core w/HT is useful for desktop tasks. But anything beyond that is pure overkill.
Maybe I'll get rid of my desktops, for a nice lightweight Trinity or IB laptop. Still considering.
I've been giving away more computers lately. Mostly single-core older machines, but I donated a couple of AMD low-power dual-cores recently too.
Perhaps I'm just learning that I've been spending way too much on computers over the last few years, chasing something that I'm still not sure what, with my upgrades.
Thinking of selling my X6 @ 3.51, 16GB DDR3-1600, 240GB SATA 6G SSD, dual GTX460 1GB OC cards, etc., to a friend. It would cost $1500 to put this rig together today from Newegg (approx.).
What a waste of money. I've played Skyrim about once or twice on this rig, total. Not into PC gaming as much as I though.
I built a nice little mini-ITX box around an Asrock E-350 board (now I wish I had picked up the USB 3.0 version). Perfect little low-power (low-heat!) NEFbox. Fine for forums, even with a HD and not an SSD, and it can watch 1080P video too. Built-in HDMI output, DVI, VGA. eSATA too. Thinking of making that box my full-time rig, at least during the summertime.
I have been big into Distributed Computing, but as the summer gets closer, I have to stop doing that on my computer due to primarily heat reasons (and running the AC at full load isn't cheap on the power bill either).