It's a good point, but Macs also are iMessage clients and are arguably less locked-down than iPhones. (I could be wrong, but not sure you can say Apple has root on Macs the way they do on iPhones.) It would be suspicious to the intended recipient if a forged message appeared one way on an iPhone, but differently on a Mac (or didn't show at all).

I am not sure about this type of security argument generally. It says, with all the guns out there who cares about knives? Well, maybe lots of people still care about them. And they still hurt when you find yourself on the wrong end of one.