In my opinion it does make sense for a few reasons:
Most people can draw a sprite, but how many can do 3D modelling?
A recent press release from unity claims to have 17 of apple's top iOS games of the year developed in unity and 1.5 million registered developers, and while some of those developers are using it for 2d, I've heard it's not all that well suited for it, so it seems like most of those users are using for 3d - and that's just unity, so I'd say a lot! There are also many options for getting content bundled with programs and from digital shops as well.
The maths gets a lot harder too, with things like quaternions instead of a simple angle.
Couldn't that be simplified by allowing the user to choose between using quaternions, 3 axes, or simply aim towards a object/point in 3d space? Like an action: object: point at object 2, or object: point at object2.x, object2.y, object2.z, or set roll to x, etc.
IMO combining this complexity with a "simple" event system doesn't make sense, because it's still complicated.
I think it makes sense - if a person wants to develop something that requires complex portions, they would still appreciate having the other parts that don't have to be complex be simpler. I was able to do a surprising amount just with the functionality cc had. A lot of game logic doesn't even have anything to do with the number of dimensions, and the event system would help with that tremendously.
The fact that playmaker and kismet exist for unity and UDK shows there's a demand for a way to make 3d game development simpler with a nice visual interface. I think c2's event editor is vastly superior to either of those and would work brilliantly for 3d as well as 2d. Besides, some of us WANT to make complex stuff. It would certainly be better if power users didn't have to leave if they wanted to do such things.
Popup 3D like Classic had in a very limited way is possible, but I'm not sure how useful it really is, plus it would depend on WebGL support in C2 which is not really ubiquitously supported yet.
I'm not quite sure what you mean by pop-up 3D, but regardless of you're talking about the 3d object, sprite distortion or simple z depth with sprites I could definitely make use of all of those even if they are webGL only. After all, we can detect if webgl is not enabled and replace the effect with something else if that's the case, and if we're targeting desktop exe it's not an issue.
3D would also open up construct to a whole new segment of users, increasing your user base and target audience, meaning more sales and c2 dominating the industry, crushing all others beneath its - I mean, uh, more sales. Yeah.