Since when were we talking about large businesses? I was talking about a "professional" with a desktop (or two if one is a notebook) and that individual being their own IT department. Of course if they have only one computer and that one isn't functioning then it becomes a problem when you have to send it off for repair if you don't know what is wrong with it. Having the original RAM around saves having to wait for Apple to repair it or diagnose the problem.
These are two separate situations. A larger business is not going to upgrade 200 computers with third party RAM. They work with the manufacturer to see if they can get some kind of discount and BTO to their specs they want or order a vanilla computer if they find that the original RAM is enough for the needs of the business.
I doubt Dell sells many Precision Workstations to home and very small businesses. These guys don't need this kind of hardware. Profesionnal Workstations are usually found in organisations with dedicated IT and service contracts.
The problem with the Mac Pro is that people still think it's a sort of prosumer machine. This gives us the XPS Studio comparisons and the "ZOMG, Apple is 1000$ more expensive for less!". The Mac Pro is now a full fledged Professional Workstation that's meant to compete with the Precision line-up. Apple does have to work on Applecare though, not offering on-site support is killer for that segment (same with Xserves, I want a field engineer to deliver and replace that disk).