I'd never thought too much about cable housing - just make sure that I have shifter-cable housing when running indexed shifters.
But I'd noticed that the shifter on my flat-bar commuter bike isn't working great. (7-speed rapidfire unit for rear derailer, no front derailer as I'm running a single chainring) I usually would have to over-shift (push the rapidfire lever beyond the indexed click) to shift into a larger cog. And I would have to push somewhat hard, to the point that after longer commuting rides, my right thumb was sometimes sore at the fleshy point that pushes into the rapidfire lever.
I thought the cases were:
a) old chain that may be more side-to-side flexible and thus shifts less crisply - chain isn't "stretched" too far though
b) rapidfire mechanism gummed up over time on a bike that lives outside, albeit under a shelter when it rains
c) shifter cable was noticeably less smooth exterior than some shifter cables
I asked a bike shop about buying new shifter cables, and the conversation with the mechanic convinced me to try new cable housing first. According to the mechanic (and this is totally plausible, I'd just never considered it) old housing gets gunk built up in it that impedes the movement of a cable. Cables get gunked up too but it's easy enough to clean them off with a solvent-dipped rag.
So I installed a new shifter cable housing, just between the chainstay and rear derailer. Shifted fine on the bike stand.
And when I biked to work this morning, when I first made a shift, I accidentally punched through three cogs! The cable friction was so much lower, and apparently the strong pressure that had been required to depress the shifter (only for shifts from smaller to larger cogs), was entirely a result of friction within that section of cable housing.
For the record, the original cable housing between chainstay and rear derailer was also too short, and my replacement is about an inch longer so the curve is less sharp.