I see where you are coming from but this is a machine that most on these forums would have no use for.
If you are a professional cad user then I guess you will understand that each new level of graphics card improves rendering time. As I don't know you could tell me how much time it reduces rendering by. I am just making educated guesses.
I do have friends that do a lot of rendering using programs like Maya and also use amazon farms because it is quicker than his multi gpu machine.
As a developer I could get some benefit from this type of machine for local build times but I would baulk at the price of having to replace it every few years. I don't build every minute of every day and for the most part I wouldn't benefit from this machine.
rendering is traditionally CPU based.. most legacy software is doing it this way.
the GPU in imac pro will allow for real-time rendering.. if/when the software developers re-write their applications to GPU based rendering.. when rendering becomes real-time, there are no more time reductions possible..
until some of the big players in CAD world adopt this type of processing, the GPU doesn't really matter for rendering times.. if they introduce the ability in 4 years from now, 2017 imac pro will be able to run it as intended.
all of that aside-- there are far more beneficial workflow/user speed enhancements than raw rendering times..
i might model a project for two weeks.. prep it for rendering for 4 hours.. computer processes the rendering for 2 hours..
if my rendering time doubled, to one-hour.. then so what.. nothing changed.. the project still took 10.5 days.
lots of stuff needs to happen prior to pushing the 'render now' button.. the most beneficial enhancements in software should be (and generally is) focused around improving user experience prior to 'render now'.
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fwiw, i mostly use autodesk's cloud rendering service these days.. doing that, you're leasing time on a supercomputer (ie- 64,000 CPU cores) instead of buying a much more expensive and much slower 40+ core local rendering machine.
like, for less than $1, i can have a full-size full-quality image in less than a minute.. and i can do this from a mbp or imac or whatever.
until the developers get real-time GPU based rendering functioning properly in their software, this is the fastest and cheapest way to go about it (for me personally at least)..
but again- it's nothing to do with ability to swap GPUs in a computer.. the hardware is already ready (or will be very shortly).. it's a software problem now.