and the fact that it is actually a robots exclusion protocol makes google's "Allow:" extension fundamentally unsound.
Allow just makes it easier to punch small holes in the firewall, albeit non-standard ones. Besides, Google added a lot of extra crap to robots.txt that many don't support. Needs to be standardized and to this day, best I know, it's not so the whole thing is moot really.
Without a script to back up enforcing robots.txt it's quite useless really.