Build features or ship it. Would you want it delayed? New APIs and controls can be added after the fact.
Both Android and iPhone have constantly added in new APIs.
Whatever the reason, for a platform that's shipping 2+ years after the competition, WP7's APIs are weak sauce. In many ways, both Silverlight and XNA are nice things to program, but you run up against the walls of that garden pretty quickly. Limited access to a lot of things that are taken for granted on iOS; the bizarre and unexplained dichotomy between Silverlight and XNA; no true 3d in Silverlight and no useful UI controls in XNA will make chosing one or the other tricky for some classes of app; no programmable shaders in XNA even though the hardware will support them; and of course no native code, meaning that porting games from iOS is off the agenda for most developers.