With that said, we are working on new methods and workflows to bridge the gap between Painter in Designer. Once such workflow would be to have a Painter node inside of Designer. This node would allow you to open Painter to create masks that then drive procedural effects such as Mask Generators in Designer. You will definitely see more workflow improvements working with Designer and Painter.
I like this idea very much. It's one thing missing for me at the moment. Rather than a "send to painter" workflow it should be a "live" workflow where you have both applications open at the same time and the "child" application shares the "result" of the parent program in it's 3d view.
So with Painter as the child application, you'd see the result of the SD node hierachy as you painted the "paint node".
With Designer as child app, you'd see the result of the SP layers in the SD viewport, say if you'd decided to "live-tweak" a generator or substance applied to a layer.
I only see one problem with this approach and it's the updates of each program. With larger node graphs in Designer and many layers in Painter, I could see this locking up a computer.
From the looks of Designer, there is way to much going on for live updates, since in reality, it's drawing bitmaps on every update, having both programs drawing, loading, redrawing I would bet this isn't possible.
That said, maybe it is but a send to would work just as easy, you do it when you are ready, maybe you don't want it to update all the time since you are just working in "one" program at a time.
On the main subject, I think it was smart to leave the two program separate, artists and node designers can be very different people and if you jammed all of Designers nodes, efx maps and such, it would get overwhelming.
I don't know if Unity is the best example to compare, Unity has "plugins" be they all feel like bolt on additions that don't enhance a workflow. Having, Painter and Designer, you have two specific workflows for two very different art creation routes.
Just my humble opinion.