Exactly why All-in-One systems are impractical for some. Aside from very few BTO options that are over priced, owners can only modify the RAM. Everything else is built in, rendering simple updates such as graphics cards impossible. The prices have increased tremendously since the PPC years, 27" base iMac's cost what a PowerMac G5 cost, in fact MORE than (I paid ~$1500 for a PowerMac G5 before the Mac Pro with Xeon Intel processors replaced it). Spending $2000+ for a 27" model that will lose AppleCare coverage in three years with little room for upgrades is ludicrous (so is daisy chaining numerous external devices, which many photographers are forced to do, defeating the purpose of a "clean and simple" all-in-one desktop).
A tower such as a Mac Pro can be upgraded, individual components swapped on-site with little to no down time, and can last longer. My 12-Core Mac Pro, which is almost three years old but is still the current model, has lasted me longer than my MacBook Pro's. I added a USB 3 CalDigit PCIe card, replaced my ATI Radeon with a newer model, added a SSD and LG Blu-Ray burner (OS X can play Blu-Ray's with third party app's), four SATA II HDD's, etc. If one of my drives fails, power down, swap it out, done. Less than a few minutes. For an iMac, you're looking at days.