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?
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.