Pre-roll ads are going the way of popups and other intrusive ads. They won't be around in a couple years. And the online video services that use them to monetize their audience won't be around either.
Because the thing you have to understand about digital media is its pervasive and abundant. There is always somewhere else to get the same thing. Digital is write once, read everywhere. Digital media is like a virus. It spreads like crazy.
So if you want to build a business around digital media, you have to be the best place to view/consume the media. Being the only place to see it is a naive strategy that won't work. You have to make digital media easy to find, easy to watch/listen/view, easy to comment/tag/share, and easy to replicate/reblog/republish.
That's the way two way media works. If you don't understand/accept that, get out of the business because you'll be out of it sooner or later.
Pre-rolls are annoying. Users are impatient. They aren't going to wait 10 seconds or more to see the video you are interrupting (because that's what you are doing) with a pre-roll. They'll just head somewhere else where they know they'll find it without the pre-roll.
Now post-rolls are something else entirely. If they are well targeted and entertaining, they'll be viewed as relevant and will be watched with interest. If not, the user will just navigate away. No harm, no foul. And they'll keep coming back to your service for more entertainment.
I am not sure about mid-rolls. I think there's a chance they will work. But I am not sure. I haven't experienced enough of them to be sure.
The people who work in this business aren't sure either. They say "I don't think we know yet what the consumer wants."
They might not know about pre-rolls. But I do. They are toast.