Do you think a re-distributor providing full source in a weird archive format is complying with the GPL?
This project http://sourceforge.net/projects/minivmac/ is GPLv2 based on the earlier vMac emulator but it provides the source exclusively in mac disk images. While after jumping through some hoops you can eventually access it on most platforms or use a Linux LiveCD to extract it and translate it to some open format it seems to go against the requirement of providing usable source code.
Even if this particular example was fine. Where does one draw the line? What's to stop a malicious vendor from distributing their modifications to busybox in WordPerfect format? Photographs of ENIAC punchcards in Deluxe Paint format? Or, ultimately, some ad-hoc format nobody can read?