A recent study conducted on only 40-odd participants in the US, age 18-22, concluded that ‘people who are well-versed in curse words are more likely to have greater overall language fluency’.
First, the sample size is too small to make this certain a conclusion.
Second, the number of profanities with which one is able to come up does not mean one uses them habitually in everyday conversation. Mere knowledge does not equal practice, it just means that people who KNOW more curse words tend to have larger vocabularies.
Third, let us assume the conclusion is accurate. This still does not account for the rest of the native and non-native anglophone world. Results may differ.
Fourth, I have yet to meet someone whose every other word is R-rated use a variety of such expressions: it seems to be the same few words. How is that a reflection of a rich vocabulary?