I have taken the 100mm macro as my only lens on mountain backpacking trips and been pretty happy with the results. The only downside I've seen is a few shots ruined by purple fringing that were unsalvagable, but none of those were probably going to be amazing anyway...
I know you're not looking for another option, but... I've been tempted to try out the new 70-210/4 lens and carry a close-focus filter (I already own the 77mm canon 500D) -- with the filter it would do 1:1.2 magnification at 210mm and be a fabulous do-it-all combo. The main thing stopping me honestly is that I'm having too much of a love affair with my Pentax primes and wanting to burn my weight there.