Workstation customers make purchasing decisions based on proprietary software features and performance, which HW vendors spend a ton of money developing and don't want to give away to their competitors. The existence of open source drivers is not the issue, it's the implicit "and then we'll ban closed source drivers" that would be a problem for workstation customers.
Workstation seems to be an exception to the typical user base, since the "Linux workstation market" is really the old "Unix workstation market", which didn't have a problem with closed source drivers.
We adopted a two driver strategy because we felt that workstation absolutely needed a closed source solution (because of the market pull for parity with Windows and the competitive challenges that brings) while most other Linux users could be satisfied with an open source stack that had all the important functionality but not the proprietary features or performance work which feeds the workstation market.
Our workstation customers felt that it would be handy to have an open source driver which could be swapped in to confirm that a kernel problem was in no way related to the binary graphics driver, but every one I spoke with (including both ATI and NVidia customers) said that what they cared about was performance and stability on a standard commercial OS distribution (typically RHEL or SLES/SLED), not open-ness or ability to work across arbitrary distros.