I've seen rag joints that were dry, brittle, and cracked to the point they were loose and hard due to shrinkage. I've seen rag joints that were saturated in oil and God knows what else, turning them gooey over time. Both can be dangerous and neither is suited for spirited driving, but I feel that a new, quality rag joint is probably fine for most people.
"New/ quality" "Probably" and "Most people" are important terms here. If I'm buying "new" items then I'm definitely upgrading. How do you upgrade a bolted in piece of tire? Eliminate it. "Probably" has ended poorly for me on numerous occasions where it seemed I was the first and only person to experience a particular problem. I can't count on "probably". And most importantly, I'm 180° out from "most people". If I can imagine a way to change something to improve it then I feel it's worth it. Sometimes that improvement comes down to vanity- "This will look cool and I'm the only person that has it" is good enough on occasion. Other times it's performance related, or even just to see if I can. I won't settle for the old rag because I like to utilize things that I can "set & forget", and I'm no fan of performing the same job twice. This goes double when new parts fail.
Of course nobody is perfect, accidents happen, and some things slip by QC so we need to keep our expectations in check. I expect to return half of the parts I get from a place like O'Reilly's or AutoZone. I confidently expect the Jeep or Astro shaft to withstand much more of whatever I decide to throw at it, and if I don't abuse it then there's no way it doesn't outlive even the best rag joint ever made. You can be gentle with a rag joint and it's still going to expire like old food some day. The simple fact that a Jeep shaft won't go bad on its own without me beating on it is a huge selling point to me.
I'll be installing the Jeep steering box and pump since that is an improvement on its own. I need to swap my column because it was the recipient of a "Chicago Punch": the guts are visible, and everything that isn't missing is loose. I've eliminated the column shift as well. Since I'm replacing that many parts I'd be a massive fool to not upgrade to the Jeep or Astro shaft unless performing a concours restoration.
Modifications that are good by themselves are often part of a bigger system that would benefit from multiple upgrades working harmoniously. Better intermediate shafts, tighter gear boxes, bumpsteer eliminators, and tall ball joints will all work together to vastly improve the steering. Their existence is reason enough for me to upgrade to them.
At the end of the day that's how I justify spending money just to add to the list of things I've modified for the sake of having a bad-*ss G Body. I may or may not need a Jeep steering shaft, but I'm damn sure gonna have one.