So because they're speculative destroying them is fine? We know they help to bring a poisoned man back to fine health, what more reason do you need to keep them around????
I can't think of any other reason to defile the ashes except for the Reaver spec (Which is useless to Rogue and Mage Wardens anyway). Maybe some people defile it for sh*ts and giggle. In my City Elf play through, I defiled 'em just to give Genitivi a middle finger.
Also most people don't know how good the Wardens are at killing Darkspawns. Wardens haven't been useful for 400 years, so to everybody your character is just another knight or fighter with some knowledge about darkspawns. Also nobody alive currently in thedas knows what a blight even feels like so how the hell do they know what Wardens can do? Especially since your character and Alistair are rookies, so they're definitely no expert Darkspawn killers or have any extra special knowledge to share with everyone.
I actually don't really think that destroying the ashes is necessarily the wisest choice. I can see both sides.
Roleplaying wise though, there is more than just getting the reaver spec, like having the favor of a cult allied with a powerful dragon. If nothing else, the deal means you can travel through the area without having to fight them (useful for not getting killed).
But my original point posting in this thread is that the Warden defending themselves against Leliana and Wynne trying to murder them (even after the ashes have been destroyed) is hardly evil. I would also say that this attempt of murder is not justified. The ashes were already destroyed and could no longer be preserved, their powers were rumor and hearsay at best at the time, and pouring dragon blood on the ashes hurts no one directly. It is essentially killing over religious beliefs and rumor. I don't think it's justified even if Wynne and Leliana were completely ignorant of the importance of the wardens (and they definitely are not). Besides Wynne when the warden sides with the templars, this is also the only point in which Wynne and Leliana attack you. It's hard to argue that their betrayal was completely a matter of morality when you can do things like blood sacrificing elven slaves with only disapproval points gained.
I really don't think it's fair to compare Leliana and Wynne to most people in Thedas. They are clearly more educated - Wynne especially has a particular knowledge of warden history.
Alistair and presumably the warden do have extra special knowledge to share - they can sense darkspawn and thus can alert others to their presence. They are immune to the blight (pretty useful if you're fighting darkspawn). They had a special link to the archdemon (the dreams of which confirm that realness of the blight).
In attacking the warden(s), Wynne and/or Leliana also have to fight the rest of the party (the warden + one/two of alistair, morrigan, sten, oghren, shale, zevran, or dog), further lessening their chances of success against the blight. Leliana and Wynne don't wait until the wardens have got all of those armies to fight the darkspawn - they attack even if the sacred ashes quest is the very first thing you do after the mage circle or lothering quests, so they could potentially be killing everyone else in the entire party over the ashes. Then it's just Wynne/Leliana by themselves.