Think I've found a reason to bump pilots all the way down to Newbie at the start of a game.
Of course its to bump others up to Skilled or higher, but it takes a lot less XP to promote a Newbie or Green pilot than it does to promote Average or Skilled. So if you lower one guy to Newbie to bump two up to Skilled you're really getting your "money's worth" in promotions.
I'll lower a Cobra pilot to Newbie cause they're cheap aircraft with low payloads so they're mainly a supporting role anyway. Keep the newbie out of the heavy fire for two missions and voila he's promoted and you've recouped half the "cost" of your other guys' improved Skills!