The resale value of Macs is certainly impressive and is the biggest thing that Windows users forget about when bashing Macs for being expensive. Even a liquid damaged three year old MacBook can fetch a pretty premium on eBay as a spares/repair unit.
If you don't need the newest graphics every year then buying a new iMac ever two or three is a great option. $2199 buys you an iMac that outperforms the equivalent Mac Pro with 27" screen at $3498. If you buy a new iMac in three years and resell your then old iMac, you will likely spend the same overall as a MP and 27" screen now but end up with a more powerful unit now and a much more powerful unit in three years time.
If you don't need the newest graphics every year then buying a new iMac ever two or three is a great option. $2199 buys you an iMac that outperforms the equivalent Mac Pro with 27" screen at $3498. If you buy a new iMac in three years and resell your then old iMac, you will likely spend the same overall as a MP and 27" screen now but end up with a more powerful unit now and a much more powerful unit in three years time.