In order to support Data Portability, as the user, and owner of our own personal watch history, we should be able to decide whether or not our data can be exported, or shared (or in this case, synced) with another party/service.
With the launch of the new Discover feature, several users are requesting the ability to sync with Trakt.
Trakt has specifically stated on their site:
It is not as simple as all that as privacy policy prevents us from sharing your data which that would require. I’m not saying we will not some day offer a way to opt into something like that but I can’t make promises
It should also be noted that in order to use the Trakt Scrobble feature (linked to above), that a user has to be a Trakt VIP customer. If users are willing to give both Plex and Trakt the money to support their services, we, as the customer, should be able to decide whether or not we want them to share data with each other; especially if one side is already willing.