Two things: 1. OS X already does this in the 'Search' option of the Help menu. 2. Menus are a useful way to discover the main features of a program. That is, the user can be in 'browsing' mode, not just 'searching' mode.
I haven't seen the OSX Help menu but I've seen this feature implemented in several other program on Windows and Linux years ago. They don't seem to catch on but I'm not sure why. Maybe it's habit or the fact search seems like more effort or the need to know the exact name for something or that people prefer the 'presence' that menus give to options. Personally I think it's great, as long as there's an option to resto…
This wouldn’t really work if every app had to implement this feature on its own, probably in different places and with slightly different behavior. Even if the behavior were consistent you still could never be sure whether you can search in the first place, that uncertainty is in my experience a good way to make sure a feature will never get used.
This seems like a great OS level feature.