This quest made me teary-eyed. Not because I was overly attached to Leandra, but because I was attached to Hawke; I've always been one who was moved to tears more from the reactions/feelings of others than from an event itself, for some reason (no matter how terrible an event may make me feel). I would have been sad over Leandra anyway, but just seeing Hawke holding her as she died...oh man.
Honestly, I'm glad that the death is unavoidable, for reasons others have stated. If there were a way to save Leandra, I really don't think most people would do anything BUT that way (short of her death meaning some odd useful artifact came into Hawke's hands as a result)
And I like that it truly does drive home the fact that mages CAN be dangerous, and sparks a bit of controversy/debate over whether it's better to let these living weapons remain free because they're actual PEOPLE, regardless of an accident of birth, or keep them locked up for the 'greater good of the populace'...and then, with that latter, of course seeing what happens when you push them too far. Because seriously, there was some pretty horrid abuses of power going on: see that one quest with Ser Alrik (I think) where it was implied that a cornered mage girl would be...abused after being made Tranquil. These locked up mages become basically a powderkeg waiting for a spark (or Big Freakin' Magic Bomb of Doom as the case may be).
Apologies for getting rambly-- TL;DR version is that I liked the quest, as sick and sad as it was, and I don't think there should be a way to avoid Leandra's death. Sometimes Hawke just can't save the day, even for those closest to her.