Murray S. Kucherawy writes:

What it says is the list should re-sign if it modifies the message (or, in general, re-sign anyway). So append whatever you want, just re-sign the message. Are you insisting that advice is defective?

Defective, maybe not.

But I don't think I would follow it for my own lists. I'd rather remove the signature and tell people who are using anal-retentive ISPs that refuse to pass unsigned messages to switch, because there are too many people out there who will report you as a spammer rather than unsubscribe. I don't want my signature associated with any such thing.