My approach has been to bring the tools to handle reasonably common situations that would make the bike unridable.
Chains will break and if you can't repair it yourself, your bike is unridable.
OTOH, a broken shifter cable won't make the bike unridable, plus cables that are maintained and replaced as needed are very unlikely to break. I suppose a broken brake cable might be a problem, but you could likely get by with just the front brakes if necessary.
So at the minimum I would suggest:
Broken chain: chain tool; spare pin if your chain needs these, or else a reusable link
Broken spoke: replacement spokes or Fiberfix spoke, spoke wrench, a Hypercracker to get the cassette off to fix a rear DS spoke
Spare seatpost clamp bolt: these do break and they are light, so why not.
Spare rack bolt(s)
Torn tire: a folded dollar bill will work in a pinch as a boot
A small adjustable wrench, small vice-grips, multitool
If you use integrated brake/shift levers on your touring bike, at least consider whether bringing a spare right downtube shifter makes sense.
Last edited by breakaway01; 03-19-10 at 01:48 PM.