When agile is done correctly you are right. But agile, like socialism, is always "perfect world" scenario stuff. All too often management wants you to release early and often and miss the "fail quickly" component.
Where I am now we're expected to release often with the same standard of QA we had with a traditional waterfall project management style. It just doesn't work, leads to higher stress, turn over and ultimately failure. Then you have the shops that want to apply agile to fucking everything from janitorial services to sales. This is the cookie cutter approach, or like my old boss used to say "Give the fuckers a hammer and suddenly everything looks like a nail"
I'm just not impressed with agile. The quality of development the last 5 or so years from every shop I've seen use it has fallen sharply.