True. And not just for climbing cliffs, either. Fences, for example, make a sling useful, unless you pass the weapon to a buddy or leave it on the ground. So do steep hillsides - going in both directions - and climbing up/into structures.
That said, I generally partly unsling my weapon when I go into action. You can point and shift the weapon better in order to accomodate unconventional shooting positions or obstacles. And it's also good practise for Real Life. You won't find anyone - one hopes - grabbing your gun muzzle and yanking on it hard during an airsoft CQB evolution, but it's a viable and effective tactic nonetheless. If your sling is looped around your body and can't be gotten rid of fast, you're likely to become suddenly dead as you are dragged forward by your weapon onto whatever your opponent has waiting for you. If you can let go, then the odds are better that you will still have your distance and your balance, he will have your weapon by the wrong end, and before he can correct that you can return the intended mischief with your sidearm. Also, despite the bungees in many modern slings, barrel-thumping some guy is a lot easier if the weapon is more or less free. Trust me :-)
-Chris