A directional mic can give quite a bit more than just a few dB of added headroom before feedback.  Take a look at the frequency response charts of the 'standard', the SM58, which pretty much has 10dB down across the board(Until you get into the very low frequency signals that usually don't exist in a vocal mic anyways if you are using a high pass filter).  Even at 125, the lowest frequency they have plotted and still exceedingly low for a vocal mic, you are 10dB down at 30 degrees off the back axis, and even at 90 degrees off still 5dB down.  That can be a huge difference on a pop/rock stage for instance, particularly if the mic stays on a stand for backup vocals etc.

Of course this doesn't get helped when you get a vocalist that doesn't have a clue what they are doing and wants to look 'cool' and cups the damn grille.

Another aspect is that some singers will work the mic specifically for the change in EQ given by a bass boost from proximity effect as well.  It depends on who they are whether this is worth it at all, but is something to keep in mind obviously.  But see my above point about people wanting to 'look cool' and not knowing what the hell they are actually doing, it applies here as well:)

 

Now this is completely correct and one of the reasons that many mics may be more suitable than others on stage, but primarily this really comes into play with monitors honestly.

         Thomas