That was *my* first thought!3. Why doesn't it go to 11?
There are a couple reasons. One is internal to blender - the data structures needed to handle threads and merge tiles. The other is a point of diminishing returns.
Contrary to popular belief, threads is not magic pixie dust you can sprinkle on applications to make them run faster. Using threads actually creates computing work - you have the original work + the overhead of creating and managing threads.
The only time this will be faster is if thread processes can be overlapped and the time saved is greater than the added thread overhead. This is the clue to why our Nexis-6 friend a couple posts north is not seeing his expected speedup with a Hyperthreaded P4.
Hyperthreads is a sort of poor man's dual core that splits a single processor. HT can speed up tasks like compiling that mix computes and i/o, but don't do much for compute bound tasks like rendering.
As an actual example for the point of diminishing returns, somewho was doing tests on a big Sun box and found that after more than a small number of threads ( 4 or 6 iirc ) render time started to increase due to the threading overhead.
So, in a nutshell, 8 threads max is a reasonable compromise for now.