Free almost always comes with a cost. A cost that probably isn’t apparent or clear at first, but a real cost nonetheless.
It can occur in the form of privacy, reliability, security, customer support, or long-term development. A good example is a number of apps that end up selling your personal info or go away entirely without any kind of warning.
If you find a product or service useful and want it to exist beyond a few years, insist on paying for it.
It will help improve it, give its owner more incentive to make it better for you (as opposed to worrying about making it profitable or breaking even), and you probably won’t have to worry about your privacy or finding reliable alternatives.
And if you’re not prepared to pay for it, do you actually even need it?
PS. Seth Godin & Anselm Hannemann give more insight into the subject.