I have half a dozen desktop PCs from 2000-4 in use in my house. I had to update the graphics card, but a Core2-Dou at 3 GHz with a decent graphics card will run virtually every program in existence. Recently I went from 2.4GHz processor to 3GHz processors, 15 bucks on ebay ;-)
I don't get the obsession with notebooks. Why do I want to spend 4X the amount of money for something that last 4 years if I am lucky. For the <1% of time I am not working from my desk? In meeting I prefer people NOT to bring a notebook, this way they pay attention and I can get them out of the room and back to work faster.
I think more and more people are not working at the same desk everyday (people working at coffeeshops or traveling) or don't have space for a desktop/don't want the extra stuff that is required for a desktop if they are living in a small apartment
Also, in companies, devices are usually swapped out after a fixed period anyway (3 years at my workplace). When you manage a lot of devices, the increased failure rates near the end of the devices' life cycle can really hurt, so it makes sense to replace devices early and sell the used devices in bulk for a few bucks, hopefully after wiping the drives. ;)
And given that you swap out the employee computers well before they break down, the shorter lifetime of notebooks doesn't matter. What matters is that your employee can take her device to a meeting room, or to a customer site.
I don't get the obsession with notebooks. Why do I want to spend 4X the amount of money for something that last 4 years if I am lucky. For the <1% of time I am not working from my desk? In meeting I prefer people NOT to bring a notebook, this way they pay attention and I can get them out of the room and back to work faster.