"...just turn over a rock that's partially embedded in the ground. Instant cathole."
Yes, my favorite technique, then roll the rock back into position... or borrow from the guy in your group who actually packs a trowel- someone has to be the moocher.
I've never really "needed" a poo trowel, taking a few extra seconds to find a stick or rock to dig a small cathole is practical enough, even in hard soil. Personally, the poo trowel is somewhat of an unnecessary item, more of a convenience, one that I don't feel the need to bring. Weight savings isn't the issue behind my decision to leave a trowel behind, rather the elimination of extraneous items from my pack, the simplification of the process, so let your style and preference of gear on the trail be the deciding factor, not the arbitrary 1oz. weight savings according to some silly UL dogma. If a poo trowel somehow enhances or maximizes your experience, then by all means, pack it!