Earlier quoted context omitted.
Yeah, it should be the absolute requirement for this kind of app on all tiers - free or otherwise.
Why? If it is a feature that will encourage people to upgrade, and/or a feature that only concerns the kind of people who buy "Enterprise" plans, then why not use it as a cost differentiator among the plans? I can see how SSL is important, but the 2 lower plans are free and a few bucks a month. I don't see a problem with reserving this to the bigger plans.
Security is never a feature. The app lost all credibility at that point.