When you are making a simple surface in 3D there’s a good option called Plane 3Point, which lets you pick a direction in 3D space and extrude from that.
But why isn’t there the same for dimensions?
I hope I missed something, because trying to do 3D dimensions in a 3D view is currently absolutely a horrible experience to me (so I always switch to a 2D view but that often involves isolating objects and other extra clicks).
+1 to this. Basically a tool that works like Distance but with the added dimension annotation. Regarding Rhino I find the whole exercise of first picking a dimension style (vertical, aligned, diameter etc.) cumbersome compared to most other CAD packages that automatically detect the type of dimension you’re trying to create.
The AutoCPlane feature in V8 can help here… But yes, this has been asked for in various forms for a long time…
Ahh, ok… CPlane also has a 3Point option, so I guess I could do that first instead of switching to a 3D view (although, again, initial testing reveals that the order in which you have to click the three points is reversed compared to Plane if you want to a horizontal/vertical dimension so again, that famous Rhino UI consistency! 
CPlanes (as well as planes in scripting/programming) have always been in the order origin, X axis direction, y axis direction.
Yeah it doesn’t surprise me, the Rhino UI was never designed with consistency in mind unfortunately.
Learn once, understand everywhere is not a paradigm they embraced.
I don’t understand what you are comparing it with…
Plane (the surface command) with the 3Point option asks you for the first corner (that is the origin) the length (that is the X axis) and the width (that is the Y axis). Where is the inconsistency?
I’ve added this thread to RH-50849 3D Dimension
-wim