Yeah, then you have the strings sawing through your zero fret and requiring more frequent level/crown jobs on ALL the frets and, eventually, refrets. The downward force on the nut (or zero fret) is significantly higher than what your finger applies when fretting a note on a "normal" fret. The narrow crown on top of a zero fret would wear out much more quickly than the rest of your frets. A bone or tusq nut is not as hard BUT it has the longer channel of the string slot to support the force. I'm not sure which would actually last longer but I know I'd much rather have to refit a new nut than level all the frets. A zero fret on a tremolo guitar is a mistake IMHO.