Why do they put some optocouplers in 6-pin packages?
By adding a resistor from the base to the emitter you can trade off speed against sensitivity, so the base connection is not entirely useless. Of course you could use it in some other kind of fancy circuit with current fed to the base from an external source under some conditions, but I've not run into the need for that. 

Since the very first DIP optocouplers were in 6-pin packages, the reasoning may have had to do with the space required internally to make the isolated lead frame connections using the methods they had available, but that's just speculation on my part. The 4-pin ones came later, and, if I recall correctly, primarily from Japanese suppliers such as Toshiba and NEC who were less committed to second-sourcing U.S. parts than to making parts that were attractive to their customers.