Actually I do often use them...

The more I think about it I think that it may have to do with what is at the bottom of the tree. I've never had slipping in a wild tree, there you have leaf cover on the ground below. On the trees where I have seen the problem there is bare ground, which is constantly wet. I am starting to think that my rope is just getting grimy on the ground, and that is causing the issue. To further this theory the two times I've used the sportline in those trees I deployed from a ropebag (I was using only one end, setting a rope for the kids.)

Dietly, how's the ground under your trees in the great white north? I bet you are climbing on ground with pine needle cover, which would be similar to my nasty dirt. Either that or my theory is complete bunk!

Carelessly spouting forth poor theories, instead of getting into the trees.
Icabod (I'd rather be climbing)