ASCII already has designated bytes for unit, group, and record separators. That aside, a big drawback of using unprintable bytes like these is they're more difficult for humans to read in dumps or type on a keyboard than a newline (provided newline has a strict definition CRLF, LF, etc)
There is in fact a reason those ASCII 'characters' should stay unprintable: the 0x00-0x1f (except Tab, CR, LF) range is explicitly excluded as invalid in a whole bunch of standards, e.g. XML.
