I think the reason is that the ROT13 is a user-side workaround to a codebase that doesn't include a spoiler feature. Any of the other appreaches you mention would require persuading one of a relatively small group of people to put in the time to implement it.
If you want to make things easier, you could enter your spoilers in a paste site. I made one for this purpose, which supports Less Wrong comment syntax for formatting:
Behold, yet another paste site!
Just enter some text, and then copy and paste the URL. For example:
That's pretty cool, but I wonder how long the site will exist. If it goes away, so do any spoilers stored there.
There's a Firefox extension called "leetkey" with a metric fuckton of features, which I have to use only the feature that lets me translate ROT-13. It's still pretty irksome. (The metric fuckton of features means I have to do more menu navigation than would be most convenient.)
It's the one I use, too. But you don't need to navigate menus: if you open the settings, you can bind the ROT13 switch to a keypress.
I tried that, but I couldn't figure out what kind of input it wanted to set the keyboard command. It wasn't in a standard symbology when I managed to get something typed in, and didn't respond normally to me typing the key sequence I wanted. Can you tell me how to make it work?
Just pressing the key combination inside the input box worked for me, although I had to make a few attempts before I found something that wasn't already used by another browser function.
Right now I use CTRL+] to ROT13 selected text, which in the LeetKey settings shows up as "Ctrl+VK_CONTEXT_MENU". It may be relevant that I use the US-International keyboard layout.
If I type Ctrl-] in the box, it just shows "Ctrl+". If I then press ] alone, "Ctrl+" disappears and it just says "VK_CONTEXT_MENU".
Weird. LeetKey's documentation is nonexistent, and I can't really imagine why it doesn't work for you. Maybe you can try directly editing the about:config results for leetkey? Googling "virtual key codes" should tell you what to insert in the entries, or you can recycle one of the default ones.
It doesn't annoy me, as long as they link to a rot13 translation site.
Come to think of it, as long as they're linking, they could just have a website where it just tells you what it is. Perhaps just have it have rot13 in the URL and use that. Does anyone know a website that does that?
rot13.com does:
http://rot13.com/index.php?text=Guvf+vf+n+grfg.
(Of course, you have to urlencode the text.)
So instead of writing "Guvf vf n grfg", one might do "Guvf vf n grfg", or maybe just "Spoiler".
The advantage of the first method is that it gives you some idea of how long the spoilered text is before you look at it, and maybe recognize the text enough so you don't have to look it up again the next time you want to read the same post. The advantage of the second method is that it conceals everything about the spoiler, and doesn't even reveal any information once you look at the URL.
EDIT: Yep, apparently the link works fine even if you don't bother replacing the special characters. May depend on the browser, though.
You could post spoilers to something like Pastebin, and just link to the page on there. Or if Pastebin is too heavyweight, I could probably make a simple web site that lets you write in LW comment syntax, super-minimalistic, in about an hour. I'm trying to develop a habit of just jumping in and doing things, so let me know if you'd be interested.
EDIT: Okay, I made one.
Sorry I'm late to the game, but yes, I happen to have made such a website.
http://www.lightning-rose.com/projects/rot13.htm
Text input is converted to raw rot13 text, raw URL, HTML URL, and UBB URL.
Here's an example of a raw URL:
http://www.lightning-rose.com/projects/rot13.htm?Yvtugavat%20Ebfr%20znqr%20guvf%21
Every time I come across one, it annoys me. I have to copy the text, open a new tab to the ROT13 site, paste the text, and click the translate button.
Compare this to something like a collapsable spoiler button-box where you press a button and it expands and expands a box with the appropriate text underneath it. Even making a [spoiler][/spoiler] tag that gave a black background and equally black text would be better than the current ROT13 solution.
Was there actually a reason for doing things this way? If so, why not just include the ROT13 translation in the javascript that'd open and close the textbox? Comments? Criticism?