My experience has been that it's endemic among people who don't care to go to the effort involved in fluency, but that doesn't really shed light on the question

My experience has been that it's endemic among people who don't care to go to the effort involved in fluency, but that doesn't really shed light on the question of whence comes the fallacious idea that "self-expression" necessarily implies imprecision. If anything, I'd argue that the opposite is true, and that successful "self-expression" relies on the precise use of language; how else, after all, to express oneself in the fashion in which one intends, than to do so as precisely as possible? (In case anyone cares, I keep putting "self-expression" in quotes because all use of language is to some degree "self-expression", which makes the phrase so sloppy in meaning as to be essentially useless, save, I think, for purposes of self-aggrandizement; there seems some inherent presumption that placing something in the category of "self-expression" renders it immune to criticism, which even if true confers no more nobility upon the subject than that afforded to the finger paintings a youngster might bring home from kindergarten.)