Is a Mac pro worth it for a user that does not specifically use multicore programs

An iMac only has 4 cores. So if you apps/VMs maxes out 4 cores than the 6-core Mac Pro is for you

Yes. Yo can use Parallels' Setting to select the # of cores allocated as well as memory. Settings>Hardware>CPU and Memory

Having arbitrary programs optimally use many cores is a Classic Unsolved Problem in Computer Science. Most code needs to be specially-coded to use lots of cores adequately. Mac OS X takes a stab in that direction by using lots and lots of tasks, and hoping they will provide better use of multiple cores than straight-line code.

You are talking of running three operating systems at once. That will always use lots of cores, because each one has so many tasks inside it. While that may still not provide Optimal usage of All available cores (the Classic problem). It certainly should be better.

The late 2013 Mac Pro is a system "all tuned up" to run many fast processors on a large Error-Corrected memory while using and cooling multiple fast Graphics cards, one of which is reserved for GPU computing. You may be better served by a used silver tower Mac Pro 4,1 or 5,1 from 2009 or later.

The six core is a really nice spot in compute power vs cost in either line. In the late 2013 Mac Pro, the 8-core is also worth consideration because it is implemented in a reasonable way (instead of the old way on two chips that gave you two separate caches that sometimes had to be reconciled). But the best deal of all is that you can change the processor chips later (or pay OWC or somebody else to do it) if you decide you need more.

RAM in the late 2013 can be expanded from large to huge without arbitrary limits.

The most compelling thing I hate about the iMac, is that it reminds me of trying to buy a Grand Piano. In a few years, you may own an obsolete instrument inside a GREAT piece of furniture, and no easy way to "punch it up a little".