Thanks to Mobilman44 for asking this question! I always thought I knew why, but, in looking it up, it was a little different than I had thought.
Quote:
Coins in denominations of less than 1 yen became invalid on December 31, 1953, following enforcement of the Small Currency Disposition and Fractional Rounding in Payments Act.
From here:
http://en.wikipedia.org/wiki/Japanese_yenSo, basically, the sen (and rin) got obsoleted.
The coin people will know this: didn't the US have fractional cents way back when? Seems like a similar situation.