I commute about 3600 miles a year and I am able to keep my gears adjusted pretty well. I do find that I get a lot of grime from rainy, muddy conditions, and I think I might be replacing my shift cables and housing more frequently than I should, probably twice a year. Would squirting some soapy water thru the housing and letting it dry before adding light oil to the cable housing increase lifetime, or is this not worth the effort?
Sad to say, not worth the effort!
However, if you are a bulk user of cable outer then you can invest in a big reel of the stuff (obviously genuine Shimano) and some Shimano cable cutters (TL-CT-10).
However, order by Madison part code from your LBS and get the brake cable outers whilst you are at it.
The inners are easy to clean up, the outer liner erodes over time and no amount of blowing soapy water through shifts the detritus.
That part is the 'XTR' version, there is also a cheaper one that should work fine.
These are good, just so long as you don't cut mudguard stays with them:
I'd guess that running soapy water through the housing would cause it to corrode faster, since you'd never be able to rinse it out well. (Very) light oiling will help.