The "don't ever let go" advice is most important when you're getting inverted. The backflip is the most obvious case. Now, I've never pulled one off, but I have quite a few half flips under my belt, and there is a huge temptation to let go. If you do that, the ski is coming down on top of you and you're going to get smacked. If you hang on, you can keep the ski off you with your arms and legs. When you hit the water the ski pushes you under, and stops at (or near) the surface. It's really not bad at all, as long as you never let go....
There are other occasions when a tactical bail is well worth it - you'll know when you need too. If you're right side up and heading for a bone to hull kind of impact, then throwing the ski away can really save you some pain. BUT it's risky - if you don't throw it far enough you're in for a nasty impact.