There are two principal reasons to upgrade annually: 1) To obtain whatever productivity benefits that derive from having the newest hardware, and 2) To have the emotional satisfaction/pleasure derived from owning brand-new gear.
It's hard to put a dollar figure on any of these, as each person is affected differently. A 10% annual increase in performance benchmarks, for example, is only beneficial when the system is doing real work. Most of us spend a lot more time contemplating than the computer spends doing heavy processing, so that 10% improvement may only amount to a few hours over the year. However, for someone doing heavy crunching on a constant basis, this is a no-brainer.
So, look at it like a rental/lease situation. IF you can sell a one-year-old iMac for 75% of its new price, you'd be paying $250-$500 (roughly, depending on model) for one year's use of the computer. Add to that the value of the time you'd spend shopping for the new model, transferring data from one computer to the next, restoring the OS and HDD to like-new condition, and selling the thing. If that was 16 hours, and your time is worth $15/hour - that'd be another $240. So, in this example, "rental" is $490/yr. ($40.83/mo.) to $740/yr. ($61.66/mo.). On the other hand, if you replaced the computer after 4 years and sold it for 25% of the original price? Your cost would be $247.50/yr. to $435/yr.
So, the added cost of upgrading annually (based on this arbitrary model) is between $242.50 and $305/yr. Will you derive that much benefit?
If it was a matter of productivity alone? Using this example, over the course of four years you'd have to save 64.67-81.34 hours at $15/hr. to justify the annual upgrade. That may or may not be hard.
Upgrading for satisfaction/pleasure alone? If you'd get more pleasure out of this than spending the same money on other things that give you pleasure, then why not?
Now, it may take more than 16 hours to fully transition from one computer to the next, the possible resale prices may over-estimate what you can get, so this estimate is probably on the low side.