Certain terms have fallen out of favor in the English language. Examples are Mohammedan, oriental, caucasian, negro, eskimo, homosexual. As far as I know, none of these terms were used as pejorative terms. There are people who don't like Mohammedans or homosexuals, but those terms are not like towel head where the term itself expresses ridicule.
One advantage to these obsolete terms is that they are often more precise than their modern equivalents. They have been used for 100s of years and well-settled dictionary definitions. You can be sure that the definitions won't change, because so few people use the terms anymore.
Latin was a popular language among scholars 300 years ago precisely because it was a dead language. Spoken languages were not sufficiently standardized.
Terms like islamicist seem to be changing in their meaning. Different people use the word to mean different things, and some of them are striving to change the meaning for ideological reasons. Whether that will be for the better or the worse, I don't know. In the mean time, I suggest using the obsolete terms if they more precisely convey the intended meaning.