Layer masks (and vector masks) should define the layer boundaries when it comes to snapping and alignment.
Currently, the mask has no effect as regards snapping. It would make more sense to align and snap layers to their visible boundaries, and make my life easier too:)
I guess I can see how this might be inconvenient to implement, but from a user perspective I see value in the request.
But it might irritate people who use sloppy Layer Masks more than the current behaviour ...
As i was just about to post the same thing pretty much... I'd like to make a modification to this idea? Why not have a button that allows you to enable/disable this feature when directly selecting the layer or vector mask. It could even show up in the layer mask properties.
Then just label it simply along the lines of "Snap bounding box to visible layers".
This would also give people more non-destructive control and allow future improvements to Photoshop to provide more functionality.
Also just have its default set up as to be disabled so that those that want it can have it and those that don't appreciate it... don't.