I see the Mac Pro as a "concept car".
The super duper hot rod that is shown off at car shows.
A bad analogy. The Mac Pro is a truck. A heavy duty pick-up truck that people use for work.
It is not a concept car. It is not a F1 racer. It is not a bragging rights car.
Apple wants people to buy the appropriate product that matches what they need done. If that is a small box or a large box is not the primary point. If there is a mismatch then there is a mismatch (Apple isn't out to sell everything to everybody).
The MBA has been more of a "concept car" than the Mac Pro for the last 4 years. "What is the minimum number of sockets people will tolerate?". "Is the optical drive necessary?". "Can primary connectivity be based on wireless?". And of late "Is a 100% SSD line-up viable? ". Up until the $999 model it sold in the lowest numbers of all Mac Models ( behind the Mac Pro even).
There is no F1 racer in Apple's line up because Apple makes products that people can get things done with. F1 cars are centerpieces in some kind of sport/entertainment experience where the majority of participates are 3rd party onlookers.
Given Intel has moved the E5 class Xeons toward the end of architecture roll out cycle it is the wrong platform to put some choices of new technology upon.
The Mac Pro may have a deep problem though if there was a significant number of folks who bought it primarily for ego and bragging rites. Fad buying like that can disappear.