[i]See, I always thought sigs were kind of bandwidth wasters -- especially if you post a lot[/i]
That's my take for images, although one-liners are okay. I can understand in help forums when people post their computer specs so whenever they ask a question or have a comment, readers have an idea.
But as far as quoting text ... well, I don't do it when I talk to my friends, either in person or on the phone, and I don't do it when I leave voicemail. If someone had signatures the same way in real life, they would probably get their ass kicked. I do *occasionally* wear a tshirt with an artist or some slogan or with a URL, but I don't go around making everyone read it.
I guess signatures are for people who like bumper stickers (like my neighbour who has a crapload of them plastered on the back of her car)

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(PS. Sorry for taking this thread off on a tangent ...)