To me it depends. Something like Disquis where they were giving it away for free I agree. However if it is a for-pay service where you are exchanging money for service, they have some incentive to do right by you, so that your money does not go elsewhere.
Good point, but keep in mind that if they're a business, then the ultimate goal is to make money. It's kind of like the formula prescribed in Fight Club which refers to whether or not a car manufacturer will issue a recall: the decision will most often be based on which result will net greatest economic yield. If the profits from doing X are more than the expected losses (say, due to customer attrition), then that's a good "business decision".
That's the risk with any SaaS/PaaS/etc, not just ones going onto your site. Anytime you pay someone a monthly fee to offer you a service, there is a risk they will change things and force you to reconsider using that service, which will then lead to having to change whatever you doing to warrant said service.
Guess it boils down to trust and if you don't, buy something you keep locally and control.
