Hi Tobias, I completely understand your point of view and concern. I’ve been involved in the WIP and using V6 pretty much exclusively for several years now. I feel like in every release of Rhino we made two steps forward and one step back: We win some new functionality but we also lose some, especially in the form of customized plugins, scripts, and 3rd. party products that stop working.
My Rhino 6 big losses:
VSR modeling and some plugins like Peter tools and Asuni’s dynamic display (http://www.asuni.com/portfolio/dynamic-display-rhino/) which it was buggy at times but when it worked it was very useful to see clipping sections that are not just a flat plane.
Also another huge disappointment is BoxEdit edit. It’s extremely slow and buggy now in V6. And McNeel doesn’t not seem to give in enough priority to fit it.
My Rhino 6 big wins:
For me it has been a massive improvement in the overall quality of how Rhino ‘feels’. I can work with massive files now in a way that Rhino 5 did ok, but struggled. Also the way everything looks in any display mode, with high antialiasing is fantastic, including the new Artic display mode and the way technical view looks now.
Also the BlendSRF tool in V6 works a lot better in my opinion. Especially hot it allows you to add/remove interactive sections and change the influence of continuity in each section by dragging control points of each section individually.
Quality of Step import/export also a lot better from a geometry standpoint. Still sucks that you cannot seamlessly import/export assemblies<>rhino layers. There’s a hack from Pascal to to blocks to layer when importing, but we have not found a workable way to export layers as assemblies. that was working at some point in V6 with a script but it also broke.
Something to keep in mind about WIPs and Rhino development.
You mentioned snapshots, and Snapshots is ok. I think you are right that it has missed the mark in some areas. For you is worksession, for me in named positions. But of course in both case we both probably have not found the time to address what’s wrong with them; and the problem with McNeel is that we have to stop doing our work to do their work of explaining how to getting a tool right. That on itself is frustrating because it’s not very efficient for us. Analogy: I never go to a restaurant where I have to look at my plate, roll my eyes, get up and walk to the kitchen and teach the chef how to cook and present something that delights me. That’s how I feel we have to deal with McNeel. 
In summary: I do have V5 and V6 installed in my machines. I run only V6, because besides all the quantifiable differences between the two V6 makes my day working with Rhino overall nicer. And that matters a lot.
I very rarely copy/paste something to V5 when I want to do a VSR or Tsplines operation, but in general I don’t miss those much (I use Modo for Subd, but if I didn’t that would be a big loss too I guess).
I hope this helps,
Gustavo