> Using profanity indicates a weak vocabulary. A lack of discipline.Lack of discipline, maybe. Though that is subjective.
Weak vocabulary, no, that is objectively not true unless someone is repeatedly using a small number of profanities (i.e. someone who uses the F/C bombs and the N & R words, for instance). Studies of online communications have shown the people who use a range of profanity also have a wider vocabulary more generally, wider on average even than those who were not seen to use profanities at all.
Of course this still has some subjective judgement involved: the studies had to define what was considered profane, and may have missed many words only considered bad in a minority of places. I'm not sure what the studies did, if anything, to account for people speaking the target language as a second that they are not fluent in. These could be important factors in correctly defining the “doesn't use them” set.
> It's biological as well as cultural.
Only because there is a biological component to the reaction to the words, which is trained by culture. This could be a programmed disgust reaction, an amused one (a small rush of relevant endorphins), or a fear reaction (where the word is a slur that is often followed by further problems like the threat of physical violence). The closest we come to a truly biological reaction might be words associated with excreta and so forth, the things they can describe carrying a biological risk, but even that is culturally informed (you aren't born knowing that shit means a form of feculence, or that feculence is considered a more polite way to describe something dirty to the point of being unsafe).