Wow this thread has gone a lot farther than I thought it would.
Here is what I was basically asking....
Is it wise to take with me a spare supply of air on dives being that I often dive in new locations with random partners and never know what mess I'll be into?
I'm an avid outdoorsman. I've been on week long hikes through the woods in the middle of no where, I've climbed rock faces and worked at a climbing tower, I've kayaked and white water rafted. And in every scenario there were multiple ways to safely get out of a situation.
I feel the same should apply for scuba diving. Sure a well planned dive "should" have no problems but its been overly explained through the 9 pages of this thread that the unexpected can be about anything. Sure on dives you have a dive buddy, but thats like saying if I'm on a walk with a friend he will instantly know what to do if I fell over with a heart attack. Not everyone is a well qualified diver and wouldn't freak out in any situation. And being that I haven't met enough divers yet to have regular dive buddies, there is even more a scare situation down there.
A pony bottle to me is a simple matter of mental security. A dive to below 60' should be 99% fine, but its that 1% that I want to cover. That one percent could be the bad air, tangled equipment, low tank pressure....