I'm not too sure. Short of gaming what task are constrained to any noticeable affect by frequency of a single core? You could say "near everything" but I'm referring to a literal constraint, something processing that you need to wait for.
Every task I perform or can think of that is CPU bound and you are waiting for could benefit from multiple cores. Apps tend to be designed that way, if its going to send your CPU into a processing frenzy its typically coded to utilize as many cores as possible to speed up that task.
The problem is that tasks that
are written for multiple cores are the minority, even if they can benefit from them. Lightroom and Photoshop for example, are primarily boosted by single core perf, while certain subtasks
may be multi-threaded like filters. Photoshop in particular is just too old. And tasks in that case which are massively parallel are already being moved to the GPU rather than being left on the CPU. And if your work tends to be of the office variety, you can stress the CPU when working in Pages/Word/whatever and those are primarily single-threaded apps. Apps like browsers will also tend to feel more performant when you have stronger single-threaded due to the final compositing that tends to need to be done all on a single thread (because UI interaction is required to be on the main thread, even if you can do some work off the main thread).
The reality is that CPUs at the moment have gotten ahead of the workloads people put on them
in most cases. Basically, if you know more cores will help you, great. But if you aren't sure, the answer is likely "no". I do have some things that would benefit from Zen or Broadwell-E, but I don't honestly do them often enough on an iMac for it to matter. And ironically, in the office, I have a Mac Pro for the one day-to-day thing where it does.
My main point here is that if we are looking at common workloads, 8-core systems are currently overkill. And I'm not convinced it would be the right call to push Zen across all iMac models, and Apple is unlikely to put Zen, and the required motherboard design changes into a single high-end model.