I think they are MORE useful on index systems, if you 1) didn't have the limit screws set right and 2) switch to friction.
I just checked my bikes and it's 50-50, but all the bikes ridden by my wife and kids have 'em. My 12-year-old daughter just loves the huge chrome one on her Ficelle.
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You know it's going to be a good day when the stem and seatpost come right out.
(looking for a picture and not seeing it? Thank the Photobucket fiasco.PM me and I'll link it up.)