I frequently edit my code in both the IDE and external to the IDE when I program in xCode (not necessary to ask why--trust me--it makes sense).

In xCode, this works fine because the file reloads (assuming it's been saved in the IDE--it warns me otherwise), if I edit it externally.

In Arduino IDE the file does not reload (either automatically or after a warning). In other words, the Arduino IDE is ignoring OS X's message that the file has changed.

Suggestion: IDE should watch for and react to externally edited files and update the file in the IDE if it changes. This is, what I would call, "standard operating procedure."