I thought about a legible signature. My reasoning was that it was too specific. When you're drawing an icon of a coffee-cup, you're drawing abstract shapes that will hit as many people with a 'coffeecup' concept. Compare this to a life drawing of 'coffee-cup on a bar in Starbucks at 9pm in Seattle on 5th avenue'. Who gives a shit who's signature it is?

By drawing an abstract shape of signature I'm letting the user fill in the gaps ('yay, it's my signature'), without forcing a specific context on his mind ('oh, it's John Doe's signature').

On re-reading this sounds like harsher criticism than I intended. What I mean to say is that showing a legible signature would more clearly indicate what the field is for.

I think it's a bit easie for the user if it was signed as "sign here" vs. "John/Jane Doe". ESP if you have an intl audience.