It's comforting to know other people are going through the same grief but most of the time I need to paste into my signature because I have nearly thirty sigs that I use as templates for messages and quotes I send out. Pasting into the top line does mean it goes where the cursor is located but only because there is nowhere else for it to go above it. I'm guessing this will only work if your signature goes at the end of the message, for me, the signature IS the message (after I've modified the content).
One thing to beware of the top line where you are typing, which is effectively outside the signature, this text might appear to have the formatting of that looks correct, font, style, weight, size etc. but in my experience text that is outside of the signature can take on a raw text format which defaults to your end user's default font on their computer. That's fine if my receipient wants to use Comic Sans but if I send a font in Helvetica, i want them to see it Helvetica (I said Helvetica to sound like a stuck up typographer but I actually use Arial as that's next best thing in a cross platform sense). One of the reasons I like to type directly formatted RTF signature.
Come on APPLE are you reading this?
Fix it before we start a revolution!
Dj