My inadvertant and highly unscientific test:

The Equipment:

Me - 26" knobbys with a couple of thousand commute miles on them, thorn resistant tubes, tire liners, no slime.

My buddy - two brand new 26" knobbys, two brand new pre-slimed tubes.

The course: 60 miles of very rough, overgrown, rail trail. Both bikes loaded with about 60 lbs. of camping gear.

Results:

Me - zero flats.
My buddy - four flats.

As far as I can tell all slime does is make a mess. However, YMMV.