Apple isn’t in the business of offering anything to Android users without getting anything in return. A value-add is something you offer only your existing or prospective customers, and that can have a lock-in effect, but only if you value it a lot. Keep in mind when iMessage launched, that it only worked with other iPhone users was a much heftier limitation because the smartphone market still had Symbian, BlackBerries, Windows Mobile, and Palm in it in addition to some very early Android models, and not everybody buying cellphones was buying smartphones yet.