I guess there are 2 cases.
1 is an axle that is likely not overstressed (but still worked hard) during heavy use, but lowering the temperature of the fluid is obviously a benefit, no doubt. The question then becomes: is the cover worth the extra expense? How many more miles while lowering the temperature prolong over fluid changes? 3,000 miles? 5,000 miles? What does that translate into your time and money? I don't know.
2 is an undersized axle that is likely overstressed during heavy use. I don't believe any kind of cover or additional cooler or lubrication is going to help in that case.
As a side question, has anyone actually seen a 14 bolt (9.5 or 10.5) fail? What breaks? I think there are a few examples on the internet, but honestly not really common if I had to guess. What's the actual fluid change on rear axles anyway per the owners manual?
Except for a fluid change on my 10 bolt truck due to a bent axle replacement, I've never actually changed the fluid on my other trucks. I bought them used, high mileage trucks, so POs probably never changed it either. One of the ones with the 9.5 has a pinion small leak...that I've been meaning to get fixed sometime.