Gonna play devil's advocate for a moment. Disclaimer: I don't work for Apple. I love their products (4 iPhones, 1 iPad, more 3 laptops in the last year, 1 Hackintosh that was an old gaming tower I built 3 years ago and never used till I put OS X on it, which I purchased).
Apple's core revenue sources have changed significantly over the years. Used to be towers were a primary source of revenue. That's most likely no longer true, and hasn't been for a while.
Take into account how much money Apple has to spend on R&D internally for their products. That's not only hardware, its also software support. One thing I've learned in the Hackintosh community is that while there's a lot of hardware that works really well, its a subset of all the hardware out there (chips, mobos, graphics cards, etc). You have to pay special attention to your hardware or getting OS X running is a huge pain in the ass, if impossible. It costs a lot of money to have good programmers to support hardware.
I have to choose to believe that the people at Apple know what they're doing for the benefit of their company. If the internal folks responsible for weighing the scales have found that the amount of revenue earned for cost of development and all for Mac Pros is too narrow, or even negative, then its entirely responsible for them to be phasing out the availability of this product line.
Yeah, lots of people want Mac Pro towers. That includes me. I'd still love to switch out my Hackintosh for a Mac Pro if just for the freedom to freely do software updates without worry. I develop about 50/50 between it and my laptop.
But for Apple, who have proven over and over again that they know where their markets are going, they may be finding little to no value in continuing the Mac Pro line.
A lot of people will counter with the argument that since Apple has so many billions of dollars, its just a drop in the bucket for them to continue Mac Pros. That may be true, but Apple isn't a company like Microsoft. They don't tend to waste billions of dollars on products that go nowhere or have little chance of success. Its true that Apple has had failures (Ping anyone?), but they are very small in comparison to their successes.
Many people reading this may be too young to remember Apple from the late 80's and 90's. They were lost and wandering in the woods. They almost died a few times. I remember going with my mom to get our first Mac in the early 90's and having to pick between 3-4 different models of the same Performa series. They had multiple types of desktops available, including the Performa, Quadra, and others. Their product skews were all over the place.
It wasn't until Jobs came back and brought a laser focus to their product lines that they turned around. He spent the last decade imprinting that way of thought on the entire company. Its vital to their future success, and our enjoyment and the productivity that comes from their products, that it continues. I would like to hope that there is an endemic fear in the company of losing that focus and going back to the Apple of old.
So if Apple has found that the Mac Pro line is no longer worth it, its entirely reasonable that they begin phasing it out. I'm choosing to look forward and see what they're doing next, and how I can use what they're building to make cool, amazing new stuff. Not looking back and complaining that old things I thought were valuable are being taken away.