Personally I agree, but I'm concerned that some of our users may notIt is I (reading by email) that need it, not them. Except those willing to in-line quote.
If it wasn't prone to drive Mark to madness I'd like to see the quoted material (but never the new material) hidden in a ... button on-line in thread context; the second level and higher hidden by the button online in message context. In digest trimmed to just first-level quote. In individual messages not trimmed at all. And of course in-line quotes unmolested in any case.
Then it could all be left in the composed message, with the appropriate remediation happening for digest an on-line readers.
Another comment, and there may be a reason this won't work, is toPart of the point is that there is no longer a concept of "off the page" -- nearly all means of reading email these days can handle longer lines. Compressing the spaces out of quote markers may needlessly reduce legibility. Not that I have a problem with that either way.