The signature is evidence that message comes from me. If I sign all my messages, I can say that I sign all my messages and possibly unsigned offensive content, which is spoofed to "come" from my address, isn't sent by me.
True, I could do that. But I don't have habit of writing offensive text and saying that it doesn't come from me.
Actually, I am, but my key is only signed by bots (see below).
You might have "import-minimal" or "import-clean" in your keyserver-options in your gpg.conf, so you don't see the signatures. If you don't have them, run
gpg --keyserver pool.sks-keyservers.net --refresh-keys 0x4DB53CFE82A46728
and signatures should appear.
NOTE: My key contains information, that my preferred keyserver is pool.sks-keyservers.net, so it's used with --refresh-keys with my key even if you speify another keyserver. This isn't the case if you use very old version of my key.
I have got signature from PGP Global Directory, it wanted only to confirm my email addresses.
According to "Locate assurer" feature at CAcert, the nearest assurer is 110KM away from me.
https://wiki.cacert.org/PgpSigning
Some mailing list software mess up with headers and make PGP/MIME signatures unverifiable at least to Enigmail. Some people say that that what those mailing lists do is completely valid. It's up to you to believe in Enigmail developers or other people.
Which mailing lists do that?
At least the following:
INLINE messages are easier to verify manually (presuming that charset doesn't cause problems).
There are many web archives and sometimes people want to verify signatures of emails, which they didn't receive. Think about Debian BTS.
K9 Mail doesn't support PGP/MIME.
Debian BTS doesn't send working PGP/MIME back in subscribtion confirmations.
In my opinion, it's easier to check did you request something with [Ðebian BTS] if it has content, which is signed with your key.
This is the problem of your email client. If you use Thunderbird or Icedove or Seamonkey, you can probably install Enigmail and that signature block gets hidden. If you use some other email client, please report bug for that package in your distribution or upstream bug tracker.
And what does that have to do with INLINE signature? In PGP/MIME you would download the same mess, but inside signature.asc file.
Because I am fed up explaining myself on some mailing lists. This page will be linked in my email signature and I will ignore every question about things, which read on this page.