Well, seeing as two people asked here's my user case.
I started off with a 1366 mobo. At the start I went with the cheapest cpu to fit in it. As prices dropped and my finances didn't I swapped it out for the next gen's "second best" (I never go for the absolute top as it's ridiculously priced).
Back in 2008, SSD's were flaky and infantile so I went with a velociraptor. In the last 3 years, I added 2x1.5 TB HDD's for storage, started with a small patriot SSD (utter crap btw) and upgraded to a PCI-based Z-Drive that goes like mad.
Similarly my ram went from 4 to 6 and now to 16GB's (10GB RAMDisk).
As for GPU's I was never a fanboy so only one iteration from a 260 to a 480.
So, 3 years down the line, I've got a 3.4GHz i7 with 16GB DDR3, a bad-ass SSD and a mediocre GPU. I can safely predict this to see me through to the first price drop of the LGA 2011-platform whenever that happens in 2013.
If I were locked in to the 2008 original spec, I'd be itching for an upgrade as soon as the SB enthusiast platform hits the shelves.
During those 3 years, I had an HDD, RAM sticks and a GPU die on me. Each time, I used the warranty rebate coupon and added a pittance to actually upgrade. Not once did I have to give my PC up for any period of time. The PC on the whole remained operational and functional (albeit limited somewhat).
On a related note, I'd rather the HW choices were my own and not some 3rd party's; for good (Z-Drive) or bad (patriot).
And to those who think, I've got money to burn, there's the salvaging.
Even if I change PC entirely, I'll still carry much of the guts of my current rig over to the next. Optical Drives, the Z-Drive and most of the RAM being prime candidates for salvaging/cannibalising.