One of my favorite writers, Ben Brooks, nails it regarding how fragile the sustainability of free apps and services can be. I feel the same way as Ben — I'd rather pay up front for an app or service, rather than get attached to something free and have it go belly up (either through going out of business or becoming a shady business).
Sadly, this is how Twitter is starting to feel. I love the service, but I'm not liking some of the decisions over there. More on that later.