It is just something we haven't gotten around to yet. We intend to comb through the engine for these kinds of garbage optimizations and make them all at once. If you already have a fix we would welcome the patch.
Our goal is to eventually get down to no garbage per frame. We want to be careful about how we achieve this because we do not want to stray too far from the original C++ Box2D which is still actively developed (and which we have been continuing to
port over to Box2D.XNA). There are other optimizations we would like to make that also need to be taken into account, such as parallel island solving (using up to 5 of the hardware threads on Xbox 360).
Overall we did a lot of work early on to reduce garbage per frame and got it down significantly from where it would be with a naive port. In terms of FPS we were seeing diminishing returns as we approached bottlenecks in the actual CPU on XBox 360
and shifted more of our focus to inlining math operations and other non-algorithmic perf optimizations.
Since we made the original port we have been working on a game that actually uses Box2D.XNA and only making changes necessary to support shipping that game (http://www.youtube.com/watch?v=uGSCrymtZOQ).
For instance you'll notice that Box2D.XNA has a "MaxDistanceJoint" which is not in the original C++ Box2D. This is needed to support a crane-like system as shown in Wayne the Brain. In terms of perf, we had no reason to make changes because
it ran at 60FPS on Xbox 360.
However, now that Wayne the Brain is wrapping up we will definitely be revisiting the engine for another round of perf tuning. We want to start considering Windows Phone 7 perf. So expect more in this area in the not too distant future.