Kevin Milligan's open letter has received some attention ([1], [2]), and that's all to the good. Kevin has been doing a good job of making the case for a strengthened and a more independent Office of the Parliamentary Budget Officer (do mosey on over to his blog for more), so the following points are largely based on ones he's already made. The reason I'm making them again is that I don't want people to start thinking that Kevin is the only one who is willing to take the time to put pen to paper fingers to keyboard in support of the OPBO.

• Although the OPBO fiscal projections have made headlines, this is only a small part of what a properly-structured OPBO can do. The Congressional Budget Office regularly makes public its projections for the effects of different policies using differing sets of assumptions, and US public discourse is the richer for it. We're seeing fiscal projections because that's almost the only thing the OPBO has been able to do with the resources currently at its disposal.
• I am unimpressed by the "Kevin Page is breaking the rules" argument. The rules are dumb and should be changed.
• An effective PBO will not usurp the role of politicians; it will provide them with the technical expertise to do their job properly. Yes, there's the risk that the PBO will tell you that your pet project is a pile of pig doots, but not because the PBO is part of a nefarious partisan conspiracy. You should accept the fact that your pet project is a pile of pigs doots and do your homework next time.