I believe that it's a mathematical truth that two planes which aren't parallel will always meet in a line.

So why can't you add the relation "point on plane" in a 2D sketch when you select a non-parallel plane? (This relation is only allowed in 3D sketches in 2017.)

I know a workaround is creating an axis first between the sketch plane and the relation plane, but it feels as a glaring omission on Dassault's part.

EDIT: Apparently picture explanations are necessary. Neither Plane1 nor Plane2 in this above image is in any way parallel to any of the default planes (nor each other), but they do intersect:

2D sketch. No relations allowed.

3D sketch, relation is allowed.

Workaround: 2D sketch, relation allowed to axis created from Plane1 and Plane2.