>>"The A300 has a service ceiling of 40,000 feet and the very highest mountains in afghanistan are around 21-22,000 feet"<<

Tue enough.  Unless you're not in an A300 but in a C-130 which has a much more limited ceiling.

>>"Anyway it is highly unlikely a raghead is going to be sitting on a mountain top hoping for a lucky shot"<<

Maybe, maybe not.  C-17's doing airdrops in Afghanistan did them from ABOVE 28,000 feet.  Depressurizing at that altitude is quite dangerous.  No other large aircraft other than the C-17 is designed to do it and even then, the crews were exposed to significant risks.  In considering the risks, the USAF chose the risk of depressurization over the risk of a stray raghead with a manpad.

And some operations other than just landing or taking off from an airport require large aircraft to go low, within range of the manpad.  Personnel airdrops is the most obvious.  Heavy equipment drops is another.

The bottom line is that sometimes big airplanes have no choice but to put themselves within range of a manpad.  But absent those operational constraints, the best defense against manpads is indeed lots of altitude.