When this was discussed on the developer list, as I recall, the decision was to decouple the API change (whether to disable PWM or not) from the optimization. I was specifically asked to incorporate the PWM disable into the macro, which I did within a day.
The plan was to quickly include the optimization with PWM disable, because it would have little impact other than simply making sketches run faster. The API change, which turned out to be controversial with Tom and others, was to be considered separately. If PWM disable was to be removed at some future date, it would be easy to simply delete it from both the optimized macro and original compiled code.
Disabling PWM takes 3 more instructions with the macro, which execute in 312 ns on a 16 MHz chip. Even with the PWM disable, the const case optimization executes in 437 ns, which is still about 10X faster.
Also, I'd like to point out this macro approach is not my preferred coding style for this optimization. I did it this way because David said he didn't like the verbose inline function approach I originally posted. Indeed that way is much larger, but it has the advantage of not touching any pin when an illegal pin number is given as the input. This macro, as currently implemented, will always write to some pin even if the pin number is too large. Then again, that's a lot better than the terrible behavior of the compiled code with a too-large pin number!
edit: on Arduino Mega, this macro will suffer the issue #146 bug on the pins which aren't memory mapped in the lower 16 I/O space. Perhaps the bitWrite macro could be modified to disable interrupts in those cases, but really this sort of thing is much easier to deal with in an expanded static inline function.