The two big problems are shitty mail clients, and people not knowing how to quote (which gets enabled by the shitty mail clients).

If someone gets CC'd later than typically because the discussion got to a point where the input is needed for the current question - and in a mail thread with proper quoting surprisingly often the quoted email is sufficient context for the added guy to jump in.

What makes a big mess out of things is the nested list of fully quoted emails with top answers at the bottom I now have to go through when getting added to figure out what the fuck they want from me.