I also rely on that behaviour. In case of fix in MG’s side I plan to make a copy of the BasicEffect code (.cs, .fx) with the current behaviour to keep compatibility.
I remember in the XNA forums people asking Shawn for this behaviour (not clamping) as in WP7 you couldn’t write your own shaders and you had better functionality without clamping the diffuse color, but this was never fixed.
I also think it’s better this way, but I can’t answer any of your questions.