Jadzia Dax was a 100% useless character to the DS9 show. Both in terms of the writing for her, and the horrible acting by Farrell. Jadzia is easily and by far the very worst character on DS9.
As for the point that she was 'sometimes' used as a science officer: 100% of those lines are completely inter-changeable with O'Brien. Could have never had Jadzia on the show, gave O'Brien all those lines, and nothing would change. Just goes to emphasize very clearly how generic, poorly-written and useless of a character Jadzia was.