First, note that two of the four questions you refer to are not asking for the solution as you state, they include the solution in the question and are asking how it fits the clue.
the SE posts are all essentially the same ("solve this clue")
So are the rest of the chestnuts here. Cryptic clues just happen to have shorter clues and, often, solutions. The biggest difference is that with a cryptic you can show some effort to have solved it yourself. Let's look at some recent similar questions:
The Magic Stick
Here's a relatively easy puzzle that's actually entirely my own creation.
Aside from brute force, how can I solve this puzzle?
[attempt to solve]
How many squares can 4 queens control?
A building with an odd elevator
Tell me, how are any of those better than any of the four cryptic questions?
they're unlikely to help future visitors (which is the whole point of Stack Exchange) as so many exist that two people are unlikely to happen upon the exact same clue
I disagree. It's not like people just pick a couple puzzles at random from all possible cryptics. Especially for the more widely distributed ones, many people are likely to do the same puzzle, and for the regular installments (e.g. the Times), many people are likely to be doing the same puzzle within a relatively short time of each other. I myself have googled for solutions (and sometimes found them, sometimes not).
the questions are trivial, easy to write
Do you think any of the questions I linked above are any harder? Copy-and-paste. Post. Structurally, the existing cryptics questions are better than most of the chestnuts we have.
and can very easily be low in quality ("solv ths clu plz kthxbai")
So can any other puzzle on the site. So can any other post on this site. If you don't think the asker put enough effort into solving it, downvote. That's exactly what that button is for. But don't just downvote or vtc the cryptics because they're shorter than other, copied-and-pasted puzzles. And like other posts, cryptic questions can just as easily be high quality.
As I see it, most of your arguments could apply at least as much to all our chestnuts. Should we ban those too?