Nailer99:
At least 2 people I know who dive ponies put their alternate on the pony and their primary on their main, so that they only have 2 second stages instead of 3- I hadn't realized that at first, so I thought it might be worth pointing out to the OP.
I've heard about people doing that, but frankly, it seems to me that basically removes much of the benefit of having a pony (especially while buddy diving). Personally, I like to have the additional second stage when diving with a buddy.
To make up the requisite completely artificial scenario, if I had a buddy's regulator blow apart, dazing him and putting him into a free-fall, and I managed to swim down fast enough to catch him, shake him to his senses, stop the descent, and donate air, and if I then discovered that we were quite deep and making a slow ascent with stops may be required to save life or well-being, and if I determined that my pony isn't big enough to make the ascent all the way from the depth to which I had to go to save my buddy's life, being able to share the air in my tank (which is almost full, since this was at the very beginning of the dive) in addition to draining the pony could possibly be a very, very good thing.
(See, I told you it was completely artificial. Is something like that likely to happen? Probably not, at least in my diving. Is it worth having one little extra second stage? For me, it certainly is -- how much drag does it have compared to the pony, anyway?

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Incidentally, as to problems that can be discovered because a pony is slung: I had a very, very small pinhole open up under the hose protector on the HP hose on my pony. Even with it slung, I didn't see anything in a bubble check. During the dive, however, I was about 85' down and saw a bubble, then two... I pulled the hose protector back and discovered the very slow pinhole leak. It was far too slow to have registered on an SPG (except maybe over hours), and it was so slow that it didn't make enough bubbles fast enough to notice it during a bubble check. It wasn't a "big deal" for the dives; I finished the weekend just monitoring it. Still, I don't like losing *any* of my emergency pony reserve; so being able to catch it was a good lesson for me. (I got the faulty hose swapped out under warranty at my LDS as soon as I was back.)
If it had been a bigger problem, would I have caught it even on a tank-mounted pony? Yeah, probably. Could I have possibly missed any potentially bigger problem with my pony slung? I do not believe I could.