Strictly on the cord choice, Mark may be right and add to that the possible combined illusion of aesthetics.

Grounded, three prong cords are generally thick, heavy and round thus they appear to be much more like the professional, heavy gauge power cord that you would 'expect' to see feeding a powerful amp. The smaller, flat look of most two prong cords just don't give you the same comfort feeling, even if the wire is the same gauge inside.

With great power comes Awesome irresponsibility.