Storm:
Forgive my ingorance, but isn't that where proper gas planning comes into play.
In principle, I agree that there should be enough dive planning to ensure that divers can come back to the boat without needing to share air. However, in reality, most dive plans are quite flexible. Also, beginners can have quite variable air consumption from dive to dive.
So it is always nice to get comfy with the idea of swimming while sharing air. You never know when you might need it.
And yes, I'll reiterate my other point - it extends the dive time, and that aint a bad thing. As I said, it depends on the circumstances. Sometimes, you'll want to come up. And sometimes, you can stay down.
Let me illustrate using your example with a pony - yes, there are a lot of good reasons why using a pony to extend your dive times is bad. And most of those reasons are predicated upon an inability on the part of the diver to manage his gas. But in practice, I could use a pony to extend my bottom time and do it safely as well - and so, I am sure, could a lot of other experienced divers here. How? By ensuring I have enough gas left in my primary tank before switching to the pony, and then heading up to the surface. I know exactly how much air I'll need to come up safely from any given depth, even with catastrophic gas failure, and I can do air and deco calculations on the fly.
Once again - it becomes a matter of using your brains and not relying on simplistic rules to guide your entire diving career. At some point, you stop needing the rules but can decide what is safe and what isn't, from first principles.
Going back to those first principles: air sharing is not that difficult. It is something that should have been mastered in OW - and of the 500-odd students I have taught, I have *never* had a single student having a problem with air sharing, be in confined water or in open water. Ever. Depending on the conditions (depth/current/vis), and given enough air in the receiver's tank (ie, not an OOA situation), it is *PERFECTLY* possible for the diver to continue their dive with virtually zero incremental risk.
Rule of thirds for recreational diving? Shirley you jest. Let's try to teach divers dive planning & techniques that are actually relevant and applicable for the type of diving that they do.
Vandit