The right multi-tool can be an excellent compliment to a good knife. Note that I said the "right" multi-tool!
Others have already covered the uses for pliers, so I won't repeat.
A wood saw is good to have in the woods, of course. Likewise, a backup knife blade doens't hurt; you can never have too many sharp tools!
Many LM models have a diamond-coated file, which is excellent for touching up the edge on your knife if it gets dull.
I've always like to have scissors with me when I'm out in the woods. Mostly just personal preference, but I do feel that some small cutting jobs are just easier and safer to do with good scissors than with a blade.
Besides fixing gear, flat-bladed screwdrivers can come in handy for prying. Better to risk a screwdriver than a blade! The interchangeable screwdriver bits on the newer Leathermans, though, just give you more things to lose. In addition, there have been reports of the bit-holders breaking; the old-fashioned drivers might bend, but I've never seen one break, and even bent can still be functional.
So, let's sum up... The ideal survival multi-tool would have pliers (of course), a knife blade (or two), wood saw, diamond file, scissors, fixed flat-blade screwdrivers.
Which models fit the bill? My current woods set-up includes an old Leatherman PST-II, which packs a lot of utility into a small, light package, although it lacks a saw. (I also pack a Victorinox One-Hand Trekker, which does have a saw.) The Leatherman Blast would work, but the file isn't diamond coated.
The ideal Leatherman model IMHO is the original Wave. Must be why I carried one in the woods for years. Besides LM, several of the Victorinox SwissTools fit the bill (my pref would be the Spirit), except for the lack of a diamond-coated file; the SOG PowerLock also lacks a diamond-coated file, and their wood saw is short and not as useful as LM's or Vic's.