The thing is that phones can be lost, robbed, broken more frequently than a Desktop. It is more a safety thing than a security thing I think.
I agree with that. In fact, my old iphone 4 stopped charging some days ago. There I had two factor authentication for some accounts with bitcoin online. Fortunately, I got to change the authentication before the phone died. I would have likely lost access to the coins if I had not realized about the phone not charging.
Not very secure.