Nowadays, building [rich] Web applications can be quite challenging, as the proliferation of Web technologies has become overwhelming and confusing. The real challenge is that many interesting new Web technologies are being promoted by various groups, and it can be quite difficult for a developer or architect to filter the practical and future-proof ones from the cool and volatile ones.
As a rule of thumb, open technologies tend to be more pervasive and longer lasting (especially for the Internet) than proprietary technologies, which tend to bring more advanced capabilities early on. Consequently, Web application developers need to be pragmatically-open, by choosing open technologies whenever possible, but also by not hesitating to use proprietary ones when required. It is not about being religious about openness or anything else, but rather about being diligent so that one is able to choose the right technology to maximize the chances of success of the target application. In other words, it should not be a personal and emotional decision, but rather a business and rational one.