That's pretty much it. They already fail to provide cross-browser compatibility for their website (Opera has long had problems with it), so this is just another way of thumbing their nose at open standards.
what are the consequences besides some facebook messages ending up truncated? Do excessive messages of this type choke up mailservers or anything else of the sort?
From TFA:
as spammers make the same mistake rather often - I've seen line length checks for Quoted-Printable implemented in antispam software.
So I'm guessing quite a few emails from Facebook are getting sent to people's spam folders.