Any question answered...

When cabling, is there any reason not to run outers all the way from the shifters to the derailleurs?

I plan to do this on my fatbike for winter resistance. Using Gore cabling. It adds weight, friction, and as Regal mentions, you lose some shifting sharpness. Totally doable though.

The more cable outer, the more total compression of the outer you get when the inner is pulled, and the less accurate the shifting becomes. In theory what you want is bare inner between fixed cable stops wherever the cable run is in a straight line.

A bike tinkerer from MTBR's fatbike forum, bent metal tubing to act as cable out whenever flexibility was not required. Looked cool.