Originally Posted by
well biked
Mountain bikes with two (or even one) chainrings, which have become more popular in recent years. These drivetrains don't have nearly as much chain wrap requirement as drivetrains with triples. Long cage is for triples.
The first medium cage derailleur I recall is the 98 XTR. That was years before anyone started running two rings and a bash. I rode that 98 medium cage with 3 front rings for a few seasons. Still have it, still works fine, nothing exploded, world has not ended, &c. Sometimes empirical data trumps the owners manual.