The problem is that the intent of a user, when they see in a form, is to fill in that form. Adding a glowing background distracts them from their task, hence bad usability.

It's much like Google's homepage fade, which was originally added to draw attention to the search box (the primary focus of the page) and remove the distractions of the other links. The problem was that it fired almost all the time, and whenever there's an animation going on, your attention is drawn to the animation and away from whatever it is that you were doing. It ends up being counterproductive to animate parts of the page in order to highlight other, static parts of the page.