You can do it in post but for some still imagery I guess it has its uses, though, using the ambient and/or incandescence makes more sense especially if you're using Mental Ray. From my observation, glow needs illumination for it to show, incandescence/ambient does not. Glow doesn't contribute to illumination, incandescence/ambient can. (you get a sad and not so believable bit of light on surfaces that are actually inside its radius AND are facing the glow source, to me that's not really "contributing"). /shrug