Would it be useful to truncate quotes (or have the option to do so in the Admin CP) at a certain point when they're copied into the editor? Say include the first and last sentence - that and the arrow link are enough that people can figure out which post it is and if they've been following the discussion they probably know anyway. Personally, I much prefer to see 'I think John's right', or 'I like John's idea of . . . '
Or a 'see full post' link / button which will asynchronously load in the full quote to the quote box.
Then if people actually feel a genuine need to paste in the entire quote, or replace the truncated version with actual specific points they want to respond to, they still can.