Do signed copies excite you? Tempt you? Delight you? Or does it not matter to you?
Signed copies don't affect the content of a book, so in that way I can take or leave them, but they do make the book as physical object more exciting. If I like a book and author, and I like to own books (and I do) then having a signed copy enhances the pleasure of that ownership. I have even stood in line to get a copy of a book signed (Ethan Canin and Sasa Sanisic are the two authors I recall doing that for). I also think that a signed copy makes a good gift. I already like giving books as gifts and it makesa book a little more distinctive. It personalizes it.