And I doubt more than a few hundred have even thought about it.
You doubt more than a few hundred even thought about it?! What? From where you get that impression? Based on what? Few hundred? Have you counted? How do you know what people think? Did you make a survey?
This is not about wanting a feature. This is about fixing a flaw.
Apple also gives you automatic categorization (filtering) by date and location, so you can quickly find any image that you need.
You can’t at all find an image based on date. The chronological order the Camera Roll provides is most basic form of organization and loses strength when your collection starts to grow and time starts passing by. After 1 or 2 years of owning a phone you will not find photos efficiently just by having a vague idea of when you took it/saved it/screenshoted it.
It has the huge advantage that it saves considerable storage
The option Apple has chosen that’s not save more space than the one people request. In fact, there would be no need for shortcuts, there would only be one copy of that photo and that would be it. Photo managing would be way easier if you had the actual photo in albums. You could delete an entire album and that would be it, instead of having to go back to Camara Roll, and find your photos one by one. An Album can photos from an extended timeline, don't you realize?
I would have to choose a folder (category) for each message; most messages could match 2 or more categories, so which should I use?
No, you don’t create a folder for each specific message; the whole idea of folders and categories is to go narrowing down the variables. From individual messages to subfolders to main folders, each of these levels has decreasing items. 1000 mails, 10 sub folders, 3 main folders.
If you need a message in two folders, you are making something wrong in your category naming. But you can always duplicate the message or copy to x folder.
When I need to find something I use the very powerful search capabilities built into my email client.
Exactly, mail inbox permits that form of organization because it compensates with powerful search capabilities. Image if you did not have those. Wouldn’t it be tedious to find your email based on chronological order alone? That’s what the Camera Roll is.
Pretty much every modern photo management app for computers works the same way also.
Overstatement to say the least.