Then people who should take a few good writing courses can be gently encouraged to do that, before spending time, energy and heartache on query-writing.
I understand where you're coming from completely. The thing is, though, when most of us critique the "bad" ones, instead of a line-by-line we say things like: "I worry about your manuscript." "Your grammar needs a lot of work." "The level of misspellings gives me pause about the polish of your story." "You need to take a hard look at your novel before querying this. Possibly hire a professional editor."
Unless I'm in a really giving mood (or I'm procrastinating) I won't do an in-depth critique on something that makes me worry about the quality of the manuscript. Like you said, it's a waste of an agent's time. If I do give such a critique on a "bad" one and I see that my suggestions are being used verbatim instead of the author truly fixing things, I ignore the thread. At that point, my own time has been wasted.