Option to enable or disable pre-roll's per client

fair point… apparently I’ve only played downloaded movies on my iOS client for quite some time – and wasn’t bothered on the Apple TV where I like seeing them.

Apparently the option you’re asking for exists on the Roku client and in Plex HTPC.
That being said, there’s already an existing suggestion discussing such an option – though it hasn’t gotten any votes at the moment. I still suggest you comment/vote in that thread in order to help us avoid distracting or cannibalizing votes from each other.