I think it's also another aspect of / attempt at lock-in.
Pre Facebookitization of profile email addresses, if you wanted to communicate outside of Facebook or even begin to exit FB altogether, you had a ready means of staying in touch with at least those FB friends who displayed an email address on their profile.
Now, you are stuck with FB to stay in touch, unless/until you and each such friend exchange a non-FB email address or other means of contact.
P.S. As an example of the former, any time a FB friend wants to discuss a health-related issue, the first thing I do is take the conversation outside FB.
Pre Facebookitization of profile email addresses, if you wanted to communicate outside of Facebook or even begin to exit FB altogether, you had a ready means of staying in touch with at least those FB friends who displayed an email address on their profile.
Now, you are stuck with FB to stay in touch, unless/until you and each such friend exchange a non-FB email address or other means of contact.
P.S. As an example of the former, any time a FB friend wants to discuss a health-related issue, the first thing I do is take the conversation outside FB.