First, any mail app that pretends to "delete" a message and does not delete it from the server -- no matter what it's called, K9, K10, K20, Thunderbird or Outlook -- has to leave something behind in its storage, so that it doesn't re-download the message again and again and again and again.
So when people talk about "storage issues", well, I don't buy that. Besides, Aqua will automatically trim any already downloaded attachments for hidden messages (but only if the user did not move them under his / her control, the "downloads" folder).
Second, yes, it's true there is currently no way to "unhide" one message and no list.
But let's look at Thunderbird -- a mail app that's years more mature than Aqua, and just might have more than one developer working on it.
If you move a message to Deleted, and then empty the Deleted folder -- if the app is set to not delete from the server -- the message will still be there on the server, but *there will be absolutely no way to see it again in Thunderbird*.
So once a messages gets on some sort of list for "we've seen this server message before, and the user deleted it from the app" -- there is no going back, at all, whatsoever.
In Aqua, you can "unhide" -- yes, it unhides them all, but at least there is a way to do that.
Now, why does moving to deleted still delete messages, even if the account is set to "hide"?
For flexibility. All three methods of deleting messages are always available, from the "three dots" menu.
And I like that. And there are users who do too.
Some don't, they just want one method of deleting messages, and enjoy having the illusion of something actually getting deleted (from the app, to save storage space). That's fine too.
To each his own...
