Asceticism gets a bad rap because it is all too often confused with mortification.
"Asceticism" comes comes from ἄσκησις, áskēsis, which means exercise or training. It means the renunciation of certain pleasures and activities in favor of hard training, physical and mental and emotional endurance, and, perhaps, a few (figurative or literal) pulls, sprains and breaks along the way.
"Mortification" has mors, death, at its root. It aims at killing off the body in order to liberate the soul. It assumes a body/soul dualism, seeing the one as evil and the other as good. Sinful flesh is simply punished.
Both the ascetic and the mortifier are saying a yes and a no, but they are different kinds of yes's and different kinds of no's.
Both the ascetic's and the mortifier's bodies may become tried and sore and broken along the way, but the one accepts the tiredness and soreness and brokenness as part of the program, while the other just aims at them directly, mistaking side effects for the cure.
One phrase I've seen used for the ascetic in both Buddhist and Christian traditions is "spiritual athlete." This doesn't mean a baseball player who goes to Bible study or a basketball coach who does some zazen. It means one who, intent on overcoming one's ego-attachments, practices hard, accepts the demands along the way and is willing to make the difficult choices. For the spiritual athlete, practice is neither an imposition nor a drudgery; rather, practice is skillfully applied, vigorously undertaken, joyfully accepted and freely offered.
Seen from this perspective, it's clear that the Buddha rejected mortification, not asceticism. The Path he set forth was one of training toward perfection rather than one of fast tracking toward annihilation.
Yesterday in teisho we heard about the battered behind Bankei got from all of his zazen. We heard about Kaihōgyō practice on Mt. Hiei. I find these to be great – and humbling – examples of spiritual athletes (though it doesn't help that the Kaihōgyō monks are called "Marathon Monks"!). It would be a shame to lose their example by dismissing them as senseless forms of mortification.