Apple Care Is Great - For Some Things
It's all about how likely is something to fail and what does it cost to fix if it does vs. not buying and taking the risk.
I think Apple Care is over priced, but they also treat you quite well when you have Apple Care on a product and I think on a good number of things it's worth it. I have Apple Care on my Core i5 MacBook Pro, both of our iPhones and my iPod Classic. These are custom devices that are not really user serviceable and if they break your pretty much SOL. They are also devices that get moved around, bumped and occasional in the case of the iPhone/iPod even get dropped.
But I have no plans to get the Apple Care on my Mac Pro (and I passed on the ACD as you couldn't pay me to use that monitor)... the odds of something going wrong on it are much less IMO than say a laptop, iPhone or even an iMac. The Mac Pro is pretty basic, a case, power supply, cpu, logic board, memory and optical drive. If your like most people in this forum when you get your Mac Pro you don't option it out with Apple but instead (smartly so) added in memory, replacement hard disks, etc. into the unit none of which would be covered by Apple care anyway. But even if you did buy all the guts from Apple (sucker) the odds of a static desktop that just sits and runs and never moves blowing up are pretty slim. Sure a logic board could fail and that would be expensive to replace without Apple Care but that type of failure is pretty rare.
About eight years ago I was hired as the technology infrastructure manager for a web and interactive design/development company. In addition to the 100 desktops and 20 servers I also had to oversee 45 Mac workstations (which we connected into our Active Directory network which was less than a blast)... In the three years I worked at that company, we had to get only three of the Mac's repaired, one was user error (left laptop on, put in laptop bag and cooked his machine), one was a goofy old Mac Cube, and only one Mac workstation had a problem and it was indeed a logic board. So out of 45's Mac's (of which 40 were workstations) so for me that is one machine out of over 40 that had an issue, and these were workstations that got used 10 hour days or more six days a week by designers and developer giving the machines quite a load. So at least in my experience the odds of something blowing on a desktop is quite low. I realize it's a risk I'll take but it's one I don't mind taking.