It has nothing to do with CAD. Pick up a copy of 3d World or 3D art. Nearly all of the computers or video cards featured in ads or reviews are workstation cards. Then read the articles and see what the professionals use. In all cases, regardless of what software they use, they will use professional video cards.

Are they absolutely necessary? No, not even for CAD. Hell, I'm in the building trade and everybody I know that uses CAD uses a standard desktop, nothing fancy. However, if you're building a computer for a specific purpose (and you have the cash - as you stated, these cards are quite a bit more expensive), then it stands to reason that you would want a card specifically designed for that purpose.

Here are the cards that E-On has specifically tested and approved:

http://www.e-onsoftware.com/products/vue/vue_10_infinite/?page=15

If you buy one of these cars, you should be good (for Vue anyway). I have Poser Pro and Zbrush. I am using an AMD 5780 and have no problems with any of these programs, but that doesn't necessarily mean that you won't...