Much of Blueprint's philosophy and syntax comes from XForms. We opted for a full declarative language because it was the only way we could effectively run on the wide range of devices out there, some of which have no scripting at all. By using declarative syntax, we can encapsulate and hide the scripting specifics. In some cases, the code could run on the phone, in other case, such as XHTML, we can put the logic on our servers. It's the perfect way to deal with the various environments and their capabilities.
The Blueprint Philosophy and roadmap highlights the benefits of declarative vs. imperative, "because it was the only way we could effectively run on the wide range of devices out there, some of which have no scripting at all". For now, only very basic XForms constructs are supported, but "In our second release of the Blueprint language you'll be able to take full advantage of the power of XForms-style MVC programming".
Although Orbeon does not particularly focuses on mobile devices, this philosophy is entirely in line with our transformations- and Ajax-based approach to deliver XForms everywhere (in our case to mainstream browsers). It is excellent news that a company like Yahoo! not only recognizes the benefits of the XForms approach, but also decides to leverage the standard instead of reinventing the wheel (like Google did with their Google Mashup Editor, which looks very much like an incompatible clone of XForms).
In short this is extremely positive for the XForms community. We can only hope that Blueprint lives up to its promises.