I've found cardstock to be essential for outdoor theatre. About 90 minutes after sundown, when the script paper starts absorbing the water from the atmosphere, it becomes impossible to write notes in pencil on regular 20# paper. But on cardstock, no problem. Also, the extra weight means it doesn't blow around as much in the wind.
Indoors, I find it isn't worth the extra work involved in maintaining a cardstock script - when the holes break on the three-hole-punched paper, I just tape over the holes and re-punch it. Works like a charm, and demands much less skill than reinforcement labels.