The reason I chose to measure curse words rather than vocabulary size is that I believed that people with an adequately sized vocabulary would be able to accurately utilize words rather than relying on curse words which are more flexible. To me, curse words seem like easy placeholders for feelings that the speaker or writer isn't willing to take the time to describe.
Testing vocabulary size is similar but penalizes people with a limited but accurate vocabulary, regardless of whether they use curse words as a crutch.
But again, assumptions and flaws, etc, etc.