I assume it's simply because there's a general hatred by NSCA guys of the Bosu, and you consider it fluff or some similar reason. But here's my take. My assumption from the start is that you have no problem with the pushup:
1. I get the benefit of a pushup but by gripping the bosu with a neutral grip it really saves my wrists. I can't do regular pushups due to wrist pain, I can only do fist pushups. Pushups on the upside down Bosu feel great on the wrists and have all the benefits of regular pushups plus:
2. The instability seems to help teach scapular control, a major problem for someone like me who spent far too much time benching for too many years and developed some 'dumb' scapulae. Doing pushups on a swiss ball does not seem to do this because it's too akward to place your hands in exactly the right spot.
3. The resistance is increased by the fact that you're pushing air into the ground instead of contacting the ground. I can bench press my weight for sets of 10 but can only do maybe 15 Bosu pushups, even less when I do them at the very end of a workout. I can bang out about 40 pushups on my fists, so the resistance is considerable.
A: To be fair I didn't actually really bash bosu push-ups. Here's what I said. I was talking about having a game plan in the gym and I said "When most people are left alone in the gym bad things happen. How else can you explain people doing BOSU push-ups?"
I agree with you on the first two points. I do think BOSU push-ups are very effective for upper body training for people with wrist and shoulder issues. As a matter of fact I do add push-ups and bosu push-ups in for people with rotator cuff and scapula issues. The unstable surface forces rotator cuff activation and control of the humerus. Plus, it puts you in a close chain which means it gives the shoulder more stability.
I'm not so sure about point three though. I wouldn't take the fact that you can do less bosu push-ups than you can bench press as a indicator that the bosu is superior. Its lack of stability makes it excellent for rehab training and not as effective for strength or power training.
I'm still not a fan of unstable training. I know that it can help synergist muscle recruitment but when we are trying to get strong and big we can't have lower muscular recruitment in our main muscles which is what happens in unstable training.
It really comes down to three questions.
For a trainer: Who's the client?
What's their issue?
What are we training them for?
For someone training for themselves What's my issues?