When I was still teaching, I couldn't hawk my books in my signature (even on my home email, because the school server was so unreliable that I gave students and parents my home email for when the school one was down). Now, I feel weird about doing it with anyone other than a known fan.
Some of my friends know that I write. If they want my books, they know where to find them. But I don't want to turn into the author who is constantly selling to people who may not be interested. I've heard about authors whose Christmas gifts to friends every year are his latest book. I don't want to be that guy.
To be clear, I'm not criticizing anyone who likes the idea, particularly not if it attracts interest to their books. I just don't feel good about it personally.