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.
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?