Actually it is every excuse. Apple are a business. They make decisions based on priorities. Their number one priority is bringing in as much cash as possible.
If that pisses off a tiny fraction of their market (and the Mac Pro unfortunately is just that now) then so be it.
Apple (Jobs and Cook) actually has said that their first priority is making great products, not making money. I think their logic is that if they focus on making great products first, the money will follow.
I don't know how long Apple will be at the top with the iPad and the iPhone, but I do know that Apple has had a lot of success with their computer (desktop and laptop) segment. Their traditional computers have been selling more and more every quarter, so I don't think they want to leave that market behind. I think Apple is perfectly capable of focusing on multiple product categories. When the iPad and iPhone craze has subsided a bit, computer sales will still be there.
People who purchase Mac Pros (creative pros, people who support Macs and iOS devices with OS X Server, scientists, software developers, etc.), are very influential and probably advocate for the purchase of other Apple devices... I am certainly a huge advocate of Apple products and services because they have done a great job of support my needs as both a musician and a technology specialist for a relatively large organization with 800 to 900 iMacs, MacBooks, MacBook Pros, xServes (which will need to eventually be replaced by Mac Pros), and iOS devices. It would be foolish for Apple to alienate and neglect people like me.
Just because you might not have a need for powerful computing devices like the Mac Pro does not mean that it is a niche product for only a handful of people and deployments.