While there are a handful of characters on the Joyo list which are not very useful (璽 comes to mind), remember that the Joyo list, matching it's name, contains kanji in common use. You might not have seen all of the kanji on the list in use yet, but in general they are widely used, and thus, "useful". There are plenty of kanji that are only used in a single word, but I think it's still worthwhile learning them if you actually want to read those words.
For example, the character you reference is used in the word 禁錮 (imprisonment). Not something you'll necessarily see on a daily basis, but an important word nonetheless (there is a simplified form of the word which replaces that character with 固, however, the Japanese wikipedia entry on imprisonment uses the non-simplified version).
I've taken and passed both the N2 and N1, and honestly, if you're just studying kanji to pass a test, you will probably do fine without knowing some of the more infrequently used characters. There's usually one or two "gotcha" questions that quiz you on a more uncommon kanji reading in the vocab section, but the reading section sticks to fairly commonly used characters.
However, if you are studying so you can actually read Japanese, then I don't think there are any useless characters, as long as knowing them is necessary for understanding the language. According to my renshuu stats, I've studied approximately 3000 characters (including the entire Joyo list), but I've yet to get to a point where I've felt that I know enough kanji. I'm still adding kanji that I've just seen for the first time in a book to my schedules, and I still have a few books that I can't quite read yet because the kanji used are beyond my level.
