You've got to look at all the fares - and B fares are not available on all routings. The DO get very variable - as these several "Y" LAX-NRT fares in September:

Y1R $2612 round trip
Y1 $1356 one way
YXR $4510 round trip
YX $2343 one way
YWR $4278 round trip
YW $2456 one way
Y $4832 one way

When you can get a LKXVFJP on the same plane and upgrade with 50k / $500, all of a sudden the "B=Y" option just doesn't do much for one. (I won't get into the discussion of co-pay or not...)