X and Q were used because they're so uncommon in English, which made them great for representing uncommon sounds. I've never seen anyone assert that X in pinyin is equal to /ʃ/, only that it's the closest sound English has.

It also makes more sense to have 1 letter per phoneme rather than "hsieh" where you have "hs" for /ɕ/, "i" for /i/, and "eh" for /ɛ/. It's a very poorly constructed system to have these superfluous Hs all over the place and "eh", in particular, is extremely anglo-coded. I can't think of a speaker of any other language using the Latin script that would put that H on the end to signify that sound to the learner, which makes for a very biased system of romanization.