I had this very discussion with my boss as a Saturday boy at the age of 15 in the local bike shop. I thought I had ‘solved’ a crappy brake issue by using gear outer and gleefully showed him my fix.
He proceeded to educate me by grabbing the brake lever with all four fingers and hauling on it with all his might, which very promptly split right in front of me in the workshop, not my proudest moment and suffice to say I then somewhat sheepishly fitted some brake outer.
I’d ridden it round the car park with no ill effect, but my few test stops clearly hadn’t put much load through it. In the following couple of decades working on bikes I saw plenty more cases of gear outer having split just from gear cable loads and fatigue or a little damage to the outer then causing further issues, and also a few cases of people trying the same thing and also suffering split cables, sometimes almost immediately, other times after a few months, but either way it’s just not worth the bother, either use spiral brake outer or proper compressionless outer.
Even if it works fine to begin with you only need to not notice one tiny crack or split in the outer casing and then on the next hard stop it can fail, and it’s not a graceful fail, it’s a lever back to the bars suddenly with no brake kind of fail. Even the best of us who check things regularly don’t check every inch of our outer before every ride...
Even if you have been lucky, or intend to be lucky
, please refrain from suggesting that other people take the risk, if you want to then fine, but it’s just not OK to suggest that others should.