A core idea of them has been that the things that make coding hard for business people are different from the things that make coding hard for programmers and I don't believe that at all. (If programmers can't stand to use it to do simple things, how do you expect business people to go anywhere with it?)
It's not that a GUI can't be helpful, it's that "GUI" isn't a synonym for helpful.
If there's a problem that can be solved by looking at a map (perhaps dataflow through components) then yes, it'd be great to present that visually.
The problem is that most of the people having this discussion know programming from movies like Hackers. They want a picture of an oil-tanker rolling over despite the fact that the code in question (theoretical code - never shown in the movie) wouldn't know what an oil tanker was, it'd simply react to one number changing by changing another. The meaning is all dependent on what platform it's installed on.
Closer to the domain my favorite example of malware is the ontology editor protoge. Every day I see freshers who are highly confused getting started with protoge who are then blown away with how easy it is to write an RDFS ontology in a few lines of Turtle with an ordinary text editor.
I am confident, having scoped many a project, that often the case is the stakeholder not quite knowing what they want. A good UI then is a good step forward to allowing the stakeholder to iterate on refining their interpretation of their own problem.
I am in the market for a decent BPM that doesn't want to charge crazy amounts and forcing us to buy support (I'm looking at you, Bonitasoft). Right now we're using Activiti it is awful. It also seems like the least bad of the FOSS offering.
Oh, and if you are genuinely interested in trying K2 before buying, I'll be they'd set up a VM for you to remote into. They did for me in '10.