Use of this word is criticised by feminist writers on usage, because it is a word marked for gender in a context where gender is theoretically irrelevant: see non-sexist language. Like many such words, its use might well be unexceptionable when it is used simply to convey two items of data about an author in a single word. The true measure of the distrust for this word stems from the situation that the use of the word is somewhat more complicated than that. The word "poetess" means more than a conjunction of the concepts of "poet" and "woman".
The word "poetess" is often used in a mildly pejorative and dismissive sense; like all the best pejoratives, it keeps open the option to deny that the person who used the word meant anything of the kind. In his Epistle to Dr. Arbuthnot, Alexander Pope wrote the lines:
Formerly, in the public mind this stereotype was usually joined with chaste bookishness of the sort suggested by the old word "bluestocking." More recently, the "poetess" stereotype is drawn somewhat differently: she strikes an earth-mother pose; she writes verse that is vaguely sensual, and given to moony oracular announcements, and couples this with a habit of enthusing over her bodily humourss. You should not call a woman who writes poetry a poetess unless you mean to deliberately call forth this stereotype.