It's not mandatory for them to show up, but if they don't, then the ticket gets thrown out. Before, officers go on their days off and get paid OT (I believe), but now they try to schedule court dates while officer is on duty so they are obliged to show up. Also, they group officer's tickets together in one court date to minimize the disruption to the officer's regular duties. However, if they are called away for a situation or are taking an emergency call, that will take priority over the court case. Hope that clears it up.
I know because I disputed a ticket and the officer admitted he was wrong, but he told me to show up to court to make sure he is actually there before I leave. He wanted to make sure that I was there to plead not guilty in case he couldn't guarantee that he would make it (due to possibly being called away for an emergency or something). Luckily for me, he made it and we shook hands and he said he'd plead no contest on my behalf and that I can go home.