How do I figure that they aren't supposed to be upgraded frequently? Well they are one heck of a machine and they can handle anything you can throw at it. A machine like that does not need to be upgraded every time a new model comes out. If you did that, and are a normal "pro" user, then you will go broke.
I work for Apple, I notice the ONLY people that buy every single time a new mac pro comes out, are the MAJOR companies that don't sell there old equipment, they are just adding to it. For instance we sell to the NHL, they buy a new mac pro EVERY MONTH. But they arent selling old equipment, they are adding to it, whether its because they are hiring more people or whatever the reason may be.
To me, and to basically all of my co-workers, it is a little crazy to upgrade to the newest mac pro every time it comes out if you are the normal "pro" user. Now dont get me wrong, if you are the type that sells as soon or right before a new model comes out then buys the new model, by all means go right ahead, I just know most people dont do this with the mac pro, they are made to last.