Good question!!! Maybe they had a tree-hugger write the maintance schedule for the transmission and didn't want anyone to change their oil!?!?!
I changed mine with approx 6,000 miles on it and noticed a slight, but significant improvement in shifting quality. I would recomend it for that reason alone. I put "Penzoil synchromesh manual transmission oil" (its synthetic and very close if not exactly the same as the stock oil)
I drive approx 30,000 mi/ year and I will be changing it once a year as a prevenitive measure.