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?
Offending/illegal content? We are still talking about text, right?
Illegal texts:
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
45 5F E1 04 22 CA 29 C4 93 3F 95 05 2B 79 2A B2
F1 90 A1 E8 17 8D 80 64 34 94 39 4F 80 31 D9 C8
7A 5F 8A 09 F8 33 F7 22 1B D4 1F A6 4C 9C 79 33
C8 72 94 CE 84 F9 CC EB 59 84 B5 47 EE C1 8D 66
45 2F 6E 40 3C DF 10 71 4E 41 DF AA 25 7D 31 3F