One thing to keep in mind for this skill, is that it is also practically free (200-800 magicka) for all builds be they magicka or stamina. If they gave us a buff and let us keep the phys/spell resist for a full 20 seconds they (a) might as well just get rid of the rune and (b) would have to increase the cost to something more like the DK one (2000-3000 magicka)

That would really eat in to the magicka return as right now it returns around 4800 magicka over 20 seconds. It would be cut in half.