I've never once had a chain failure, or even an issue, on my own bike. But
several times I've come across folk stranded on the road due to a mishap
that only a chain tool could fix. Today was one of those days.
At the base of Tunitas Creek (Northern California) my son and I came across
a group of three or four cyclists at the side of the road, and I asked, as I
usually do, if they had what they needed (works much better than asking if
they're OK, because rarely, it seems, will guys admit that something's
beyond their ability to deal with, but they have no issue admitting that
they need something, probably because that implies that it's in the realm of
an act of God that has them stranded).
Well, this guy had somehow managed to do a number on his rear derailleur and
break the replaceable dropout hanger. His bike wasn't going anywhere, as the
chain, without a derailleur attached, was going to be dragging on the ground
if he were to put it on the smaller chainring so he could get up the grade
ahead. That's where the chain tool came in. I was able to shorten his chain
enough to get it onto a front/rear combination that would allow him to make
it up the hill, which he did.
Other times I've help people with failed connector pins (fairly common) and
once with a chain that, due to a really nasty shift, had gotten bent
severely and had to have a few links removed.
It's not a bad thing for somebody in a group to have a chain tool. For what
it's worth, the one I use is the Topeak Hexus (used to be called the
Hummer).
You can see a picture of the "repair" here-
http://www.chainreaction.com/images/busted_hanger0139.jpg
--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com