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In a message dated 99-01-15 15:16:27 EST, you write:

<< 
 Why is the single w/pony better than a single w/Y valve? (Let's say
 the diver owns his/her cylinders so employing a Y valve is an option).

You can do all the dive planning and proper gas management that you want, but
in the end, all a Y valve gives you is another regulator to an empty tank.
Where is the redundancy in gas supply with a Y valve? In the real world, shit
happens, reaction times are not always the same as when training and you know
the situation is occuring before it occurs. Lets say your down to 1500 psi in
your single tank, you are 130' and your backup reg on the Y freeflows, ever
have a free flow at 130'? As you realize your situation, organize your
thoughts, extract your arm from the hole you were digging in, find the valve
and shut it down, your tank has already completely drained to ambient
pressure.
Now, what would you prefer, another dead reg to suck on, and hope your mad
dash to the surface works, or a nice full 40 cu/ft pony bottle to breath on,
situate yourself and calm down, swim back to the anchor line, make a nice slow
ascent and do a safety stop. 
Failure points have nothing to do with it, redundancy has everything to do
with it. 
 
<<The only failure point advantage of the pony would be the burst disk
 and neck o-ring. Neither has a high failure rate. The failure mode
 for the o-ring, when it does fail, is usually a slow leak that in no
 way would require a bail-out. We all know how to make the risk of burst
 disk failure zero.
 
 If you're willing to discount these advantages of the pony because of
 the low likelihood of failure (the sort of calculated risk we take all
 the time), then all you're left with is more gas. Then I think gas
 management and dive planning are the proper tools, not the pony bottle.
  >>
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