Grommets are fitted to prevent water ingress & to protect the cable from chaffing....
Saying a cable doesn't move proves nothing, each & every shock through the car will cause some movement of each item, (cable & bulkhead), it only takes small amounts of vibration to effectively saw through the cables protective outer sheath....Whether it takes 12 months or a few hours is also a mute point, the fact it could happen is enough to promote all car manufacturers to use grommets inside & outside your car....
As stated a correctly fused cable is basically safeguarded against a dead short, however the same cable suffering a bad connection could easily promote enough heat to effectively melt its outer sheath without blowing its fuse, (this can also happen with short circuits), and Mr Insurance Claims Adjuster will love Bad Wiring practices....
If you wish to wire your car as a cowboy would then thats fine, but to advise third parties to follow suit is a tad irresponsible....Unless your prepared to cover any costs incurred in the event of fire....
