Officers are judged by the number of arrests they make not by the number of convictions (as conviction is the county attorney's bailiwick, and besides all cops know the courts are just a impediment to justice (almost a joke, folks)).
Officers also know that unless their actions are egregious then can claim 'good faith' and 'I Thought that was the law" an an excuse for an arrest that isn't prosecuted.
Officers also know that an arrest is almost as good as a conviction in terms of just absolutely ruining someone's otherwise perfectly good afternoon and 'teaching them a lesson.'
I agree that unless there is something we're not being told that the disorderly conduct charge will be nolle pros'd. It won't matter, the individual was still arrested, transported, fingerprinted, served food worse than airlines have and was inconvenienced. Sometimes that's all that matters.