If people care enough to read the messages, they will use an old client or parse the new format.
Sure, but if you only need a few keystrokes to make possibly offending/illegal content readable on any (*nix) Bitcoin system without having to download any special program this might become a problem for Bitcoin and justify obfuscating blk*.dat.
As soon as you need additional tools to unlock the content, these tools themselves become problematic and can be blocked/outlawed much more easily and with less harm to Bitcoin itself.
Why take the chance?