Putting your only female dev in the position of representing your company at conferences suggests that she's taken on the role of "token female". If this is coincidental, and she just happens to be the best spoken developer with the least stagefright, then clearly there is no problem. However, if you deliberately chose her because she is female, then you are doing her no long-term favors, and are engaged in sexist business practices yourself.
new_corp_dev put it better than I could: "If the industry sees a glut of women speakers who are there only because they are women, then the industry will have no choice but to acknowledge their token status."