Part of the problem is in EXPLAINING the new functions.
By that logic, no improvements would ever be added.
David Mellis specifically requested this code, and when I wrote it for normal Arduino boards, he specifically requested it implemented for the Arduino Mega, which I also did within a matter of a couple days. Difficulty of documentation was never a concern when he (and others on the developer list) wanted this, back in November 2009.
Since then, it's sat in the issue tracker for about half a year. However, he did recently flag this for the 1.0 milestone, so maybe it'll actually make it into the official Arduino core within the next 6 months?
Still not known is if this code will simply be used as digitalWrite(), or if a new name like digitalWriteFast() will be used, or if David will end up implementing it some other way. However it David ends up using this, assuming he ever does, I'm sure once it's actually committed to svn and due to be released, somehow explaining/documenting it really won't be a big deal, and if it doesn't introduce a new name, perhaps no documentation changes will be needed at all?
I've heard this "but we'd have to document it and support it" line many, many times before, usually in the corporate world by mid level managers who just don't want to do anything innovative, unless the directive comes from those above them. There is some point to it for dramatically new products, but really, in cases like this where the feature is just a performance improvement that carries virtually no risk, virtually no backwards compatibility, and is pretty much just invisible, I just don't buy that line about how difficult documentation is. It'd probably take less time than we've sent writing all these message in this thread!