Not so hard to imagine: Say you transitioned from steel line to AN using a tube adapter just after the elbow at the trans. That steel line is now supporting the AN line, unless you tie it up somewhere. Then beat on it around a track for 24 hours, hopping curbs, etc. It may not fail at all, but why take the chance? If you're gonna go AN, do it all the way.

1988 CVLX Formal Roof (Love Boat) POTM July 2019 & Aug 2020, 56K
E-code lights w/ relays, LEDs w/ switchbacks, 15x8 Ansen slots w/ 255/60 BFG's, Kenwood stereo, various hacks.