GRID: If I want unlimited Recs to display, why must I set a Height

Selecting Unlimited...

shows this popup:

I don't understand why this requirement exists.
If you specify 500px, which at best is a rough guess and will require fiddling around several times to make it fit, then you've crippled users that have bigger displays as their grid will fall short of the bottom of the screen.

I've never understood why a GRID cannot just be "100%" of the height.