The original idea was that the powder measure would remain installed in station #3, and you would just dial in the new powder charge on the measure when you change calibers. This way, you didn't have to worry about moving the powder measure from die plate to die plate, nor did you need to have multiple lower powder die assemblies to accept the measure on each die plate.
Lately, with the introduction of the bullet feeders, it's become common to move the powder measure to station 2, where you can expand and powder charge using the RCBS powder expanders. With that, I agree, it makes less sense to have a fixed station #3. Using the powder measure with the powder expanders, the depth of the powder die becomes a critical measurement, so it's advantageous to have a powder die assembly on each die plate, so that you don't have to adjust the depth each time to get proper case mouth expansion (very critical if using a bullet feeder).
What I've done on my press was adapt the Dillon powder check system to the press. I leave that permanently installed in station #3, and do all my powder checking there. Each die plate now has a lower powder die installed in station 2, pre-set to the correct depth, and with the correct powder expander and spacer (if needed), so moving the powder measure from die plate to die plate takes only seconds. Once the measure is dialed in to dispense the correct powder charge, setting the Dillon powder check rod also takes but a few seconds. It actually works very well: