A disadvantage of using << is that you lose the SGML-like syntax.
You might not consider this important, but I thought I should mention it;
it was certainly something I liked in earlier MIME drafts.
On the other hand, with << and the newline conventions, you lose the
problem trying to distinguish a command that needs a matching </> command,
and one that doesn't. I think one of the early fears raised about richtext
(by an SGML person I believe) was that if further unmatched commands were
added to richtext at a later date, then older parsers wouldn't know how
to distinguish unmatched commands from matched ones.
Various proposals, using different notations such as "<$" (or something like
that), were put forward to distinguish the two kinds of commands. "<<" was
the cleanest alternative to "<lt>" that was proposed, and fits in well with
the usual computer tradition of doubling special characters.
I hope this helps clarify some of the discussion that happened months ago.
Cheers,
Rhys.