Two comments:

MeanGene:
If you're worried about impedance when using two bookshelf speakers as a center, try wiring them in series, which will double the resistance seen by the amp (compared to using one speaker). I do this with a pair of Celestion A1s used as a center. I had no choice since these are 4 ohms each, and wiring in parallel would have resulted in a seriously problematic 2 ohm load. The center level had to be tweaked upward a bit to attain balance, however. I personally favor using a pair of bookshelf speakers as a center since the possibility of obsolesence is much less.

audvid:
Another possible tech reason for the impedance selector switch is to select between output transformer taps optimized for high or low impedance loads. Basically, the sound is taken off the point in the transformer that matches the transformer impedance to that of the speaker. This sort of thing is pretty standard on most tube amps and some costly solid state amps, like the McIntosh models that use output transformers. Strikes me that it's unlikely that anything Japanese uses an output transformer at all, much less providing multiple output taps.