Network effects. Even if my friends' iPhones are "compromised", if I can get them using the same messaging protocol, then that messaging protocol is more useful on my "non-compromised" device.

If we went with your way of thinking, most of my friends would never use Tox, thus making it useless to me, thus meaning I'd have to use a non-end-to-end-encrypted messaging protocol such as SMS or Facebook Chat.