Even when I have c-plane and c-plane handle displayed, the handle is greyed out:



Only if I have a viewport state activated, the c-plane handle is available:



Is this... really by design? It doesn't make a lot of sense to me, as the construction plane doesn't "belongs to" any viewport state.

Well, perhaps the construction plane 'belongs' to the construction plane state : )
Personally I don't use the CP handle option available in the CP menu, I just toggle in / out of it with the forward slash key "/" - that puts you in construction plane state with the handle being active.

If it was permanently active it how would you, say move a point on an object if the construction plane was centred on that point?

raincole
Only if I have a viewport state activated, the c-plane handle is available

It's only greyed out if the selection tool is active. It becomes active if Move, Rotate, Scale or, like you already said, Show Handle (viewport state) are active. I didn't experiment with the Pose.

My guess is that it's like that by design. I've never thought about it much, because I've never perceived it as a problem. It's just how it works, so I guess it's best to accept it as it is.

The main problem with c-plane in my opinion is the unavailability of the "tangent plane snapping" function, which is available for a regular plane. And instead of logically using a temporary c-plane, you have to create a real plane, position it, and snap to it. Which is longer, more inconvenient, and is a function for c-plane, in fact. This is a strange situation.

The lack of "Align Handle" submenu in C-Plane context menu is a problem too. What's the point of a C-Plane if I can't align it with selected primitives? Up vector should also be exposed for adjustment one way or the other. Just saying.

Also, my another wish is that if C-Plane is enabled, viewport snapping (to front/left/up/bottom/right/back) respect C-Plane's orientation. Or maybe it's already possible to accomplish that, but I'm not aware of it...?

ajz3d
Also, my another wish is that if C-Plane is enabled, viewport snapping (to front/left/up/bottom/right/back) respect C-Plane's orientation.

Agreed 100% on this - I believe I filed an RFE for this a few years ago...
The only single view that does respect CP orientation, as far as I know, is dropping down the 'Perpective' menu > 'Home' > 'Construction plane normal'.

ajz3d
The lack of "Align Handle" submenu in C-Plane context menu is a problem too. What's the point of a C-Plane if I can't align it with selected primitives?

You've got quite a flexible 'Align state' - semi-colon - that can be used with the CP.
But I do agree that Sidefx could make CP usage much better. It's an important tool in my book.