The problem with fractional_even is that you get a very tight mass of triangles mid edge, and very large triangles in the corners. Obviously this has potential for artifacts. With fractional_odd this difference is much less pronounced. I'm not sure where anyone would ever want to use fractional_even unless you had very specific topology that benefitted from that imbalance somehow. Here are some wireframe results:

fractional_even:

[attachment=15320:PartitioningEven.jpg]

fractional_odd:

[attachment=15321:PartitioningOdd.jpg]

Backface culling ? You need to give it some bias and you can actually eliminate the patch completely (so it doesn't get rasterized at all) by setting the factors to zero:

This works great for me unbird. Thanks for the tip! I have to set my bias kinda high, but I'm still getting rid of enough patches for a noticible frame rate boost.