You reminded me of something I read in an old spiritual book about some monks in a certain monastery. The thin ones who drove themselves nuts with excessive asceticsm were very proud because of the austerior asceticism. The fat, round, well-fed monks were humble, and the book's point was trying to highlight the dangers of spiritual pride that comes with such practices. Jesus said when we fast, it should be as much a secret as possible. That way we'll be safe from spiritual pride. So its certainly not a measure of spirituality, and someone can do that and inwardly be rotten if they full of ill intentions. But it also has benefits, which is why we are asked to deny the flesh and to fast.
I think the logic behind it is to subject the flesh or the "old man" as Paul called it.
It can be interior like lazy cowards like me prefer it (though I hardly ever do it these days...I really should start) or exterior, like the desert fathers and people like St Francs did it. Personally, I've never been able to practice asceticism exteriorily much, though I try to deny myself something during lent. But from the little experience I have, I can say there is much spiritual benefit that can come from denying the flesh through fasting or some other means of ascetism. There is inner freedom that comes with it, or a sense of not being a slave to the flesh. One lent, I lost an old addiction of mine coz of not touching the bad habit for weeks. The flesh leads many of us to do stuff we don't want, so denying it something it wants once in a while is not bad. I have no problems with asceticism as long as it is kept within reason. Doing what Filipinos do on Good Friday is down right crazy to me. But fasting and other means of self denial, especially secret interior ones are very beneficial. A good one to try is to not insist on your way even though you know you are right or even if you may be wrong. I say that one, because its very hard for me personally and I can count on one hand how many times I've been successful in doing it. But done with the correct interior disposition, it can be freeing to inner attachments to ego and addictions, which can hinder not just how we relate to others but even how we relate to God. Losing attachments to one more thing only makes more room in your heart for the indwelling Trinity. I find it easier to give than to self-deny, which probably means I should self-deny a bit more. But asceticism is just one way to grow spiritually by slowly ridding oneself of attachments, but if there's no prayer, no charity, no obedience, its fruit is nil.
The goal of the interior life is not asceticism or any spiritual practice, its in the growth of the life of the indwelling Divinity in us, until we reach that place where we can honestly say
"No longer do I live, but Christ lives in me". The life of God in the soul starts like a seedling, but nurtured with prayer and humility, it can grow more and more and transform us completely into images of Christ. The measure of authentic spirituality is inner holiness which only God can see. It can be seen exteriorly in the fruits of a holy life- patience, gentleness, love, humility, meekness, being long-suffering etc. but apart from that, it is a secret known only to God. Just that he allows some peoples' inner lives to be seen by the evidence of the fruits of the Spirit. So prayer, fasting or anything else we practice is not the goal, but the hidden interior life with God at the centre of our souls is the goal. That's why Paul says the three theological virtues are most important, faith, hope and love, but above all, love. Because these three are how we relate to God inwardly, in that secret room in our souls, where its just him and us.