OK, what criteria do you evaluate a carrier on?
As with the Sherman vs. Panther example I mentioned, by the criteria employed by the people of the time who were using the carrier in question. Just as with that very similar tank question, I'm using the what was perceived and believe then and not what we perceive and believe now.
I said she'd be useful...
I think so too and primarily in the role the USN used her for nearly the entirety of her career: training.
... and pointed out that her deficiencies compared to the USN fleet carriers are often absent when compared to the IJN fleet carriers...
Her deficiencies compared to USN combat carriers and IJN carrier's deficiencies compared to USN combat carriers are of no consequence. All that matters is what the USN of the period believes her abilities are. They're not judging her against IJN decks, they're judging her against what the USN needs and what the USN can risk.
What the IJN needs and can risk are completely different questions, so comparing Ranger "straight up" to IJN decks to determine how well Ranger can meet USN needs and face USN risks is of no use.
Are 8" guns vital for evaluating a carrier?
For the USN apart from the
Lexingtons, no. For the IJN with regards to that deck, yes. And for examining whether the USN would use Ranger more often in more dangerous roles, it's of no consequence whatsoever.
CVEs were often built from merchant hulls. Gee, they must have been really useless, right?
They were insanely useful according to the criteria of the people judging them at the time and their criteria are the only ones that count.
You're also ignoring that the US used Ranger in the OTL as a CVE twice and both times in very low threat environments because, apparently, Ranger didn't meet the criteria at the time to be deployed as a CVE.
So if the IJN carriers (BTW, the Hiryu/Soryu class had their own hull weaknesses and CV Shinano's Yamato-type hull didn't keep her from being sunk with embarrassing ease) were successful in combat despite having every weakness you ascribe to USS Ranger...
You
cannot compare Ranger's flimsy construction with the IJN carriers you mentioned.
... why couldn't she have been useful in non-combat missions that would place lower demands on her?
Why can't you understand that all non-combat missions aren't equal? Threat level encountered during those missions was taken into account by the people of the time and must be taken into account by us now.
Ranger served as a CVE, a training vessel, and an aircraft ferry primarily in the Atlantic where the naval threat level was tiny compared to the Pacific. She didn't even enter the Pacific until mid-44 and then operated solely off the West Coast with exception a few months at Pearl. Not only was she was the only pre-war carrier never to face the Japanese but she never got with a thousand miles of the Japanese.
If the USN of 1941-45 never even attempted to use her in even a combat support role in the Pacific, why would they do so routinely in 1938? While the USN's perception of her wouldn't have changed between those two periods, the USN's needs will have changed. The question now is how much those needs have changed.
The USN wouldn't even dream of putting one of the Cimmaron-class AO's in combat - but they weren't just useful, they were vital.
Which, of course, is why they routinely steam at the center of carrier battle groups.
They may not be fighting, but they're right there where the fighting is occurring. Threat levels.
BTW, she also functioned as an aircraft ferry several times.
Once in the Pacific between San Diego and Pearl in late '44. There's big difference between that run and Wasp flying of fighters to Malta or CVE working as ferries in the western Pacific.
Explain, please, how she wouldn't be HIGHLY useful in that role?
She would be and she'd be used more often than in the OTL. She wouldn't be the first choice however.
So you can arbitrarily say USS Ranger wasn't useful...
I said no such thing. I said the people of the time perceived her as not useful for many of the missions you were assigning her.
... but I can't use hard numbers in comparison to useful ships to argue that she would be useful?
For the same reason using numbers alone to compare and contrast Sherman guns vs Panther guns and Sherman armor vs Panther armor doesn't prove anything about the Sherman's usefulness.
Numbers are the beginning. Analysis is the goal. You've posted plenty of the former and none of the latter.
Again, the USN IOTL did use USS Ranger as an aircraft ferry, training ship, and as a combat carrier.
Again, not where you think they did, not in the manner you think they did, and not facing the threat levels you think they did.
What is different in 1938 that she would not be assigned these duties?
She was assigned those duties in 1941-45. Those duties aren't what you want to believe they were.
These were useful missions in 1941. How are they not useful missions in 1938?
They were useful missions in 1941-45 and they were performed far more often by other ships. They will be useful missions in 1938 too and, while Ranger will undertake them, she'll perform fewer of them in fewer places than you believe.
Ranger will be used. Ranger will not be used to the extent that you presume.