To put it in perspective for US folks, -20 C is -4 F. It's manageable, including ice, with studded tires. With that said, I don't push it on my friends. I'd rather encourage them to make riding in nice weather a habit before taking the plunge. Also, there's more to it than even cold and ice. The roads become effectively narrower due to piled up snow. It gets dark during commuting hours, making it more dangerous if car traffic is involved. Road salt makes a mess of bikes, so it helps to have a dedicated winter bike that can take the brunt of it.
