<p><pre><code> They serve no purpose, except proving that file formats not starting at offset
0 are a bad idea.
</code></pre>
Au contraire–they are a wonderful idea. If, say, a polyglot pdf+executable could gain code execution enough to execute itself as a binary, it could just as easily execute far more dangerous code while being far less likely to tip off an automated scanner. So it goes for many of the other stackable formats.<p>The benefits seem to far outweigh the drawbacks. As an example, look at the concept behind ext[234]fs migration in btrfs (which is not fixed to block 0 in a volume)–Take an ext4 filesystem, generate new inodes pointing at the original b-trees, and away you go! You get a filesystem that is valid ext4 (retaining the old data), and valid btrfs (which, being copy-on-write, doesn't destroy the old data.)