Adding a phone number to your account and enabling two factor auth aren't the same thing in my mind. It might be that I'm paranoid about giving LinkedIn my info plus bad copy on the prompt, but I had the same reaction as the grandparent.
Until someone clicks the "Add people you know from your contacts" feature that most of these viral products end up implementing.
At which point they have an extra dimension to help their social graph or whatever they want to call they set of marketing information they sell.
I realize what 2 factor authentication is, and perhaps I jumped too soon before considering it, but with everything else happening I can't help but think they would want my number for more than just my added security.