The problem is not about getting "a bit more bang for the buck", it's about making the purchase a good long term investment.
With a PC (or the rumored upcoming mac pro, or the old mac pro towers) your money lasts longer since you can upgrade and repair the computer for a very long time.
Another aspect is performance. The iMac Pro is not a Land Rover, it's an SUV geared towards weekend camping enthusiasts. It cannot be repaired or upgraded easily, and the all in one design doesn't allow you to use it's full power since
it cannot dissipate all the heat it generates.
If you just need more performance than a regular iMac but you won't be abusing that machine the iMac Pro might be a great choice. OTOH if you are going to leave it rendering regularly for days, even weeks, where 10% of performance will make a difference, it might not be the machine for you. Also a machine that is being constantly abused will fail at some point, and some component will need to be replaced. It's expected, which is why Land Rovers are easy to fix.
Don't get me wrong, I have a 27'' iMac myself and I love it, but my performance needs are low. I don't need a Land Rover, I'm fine with an SUV for my suburban life. But people that need to cross difficult terrain daily they need a Land Rover.