Hm jeah for me personally it makes no sense to only mark videos as watched when I watched them until the end. The only point where this could make a little sense is for movies in my opinion. The problem I see is that we don't have an indicator for "partly" watched videos. So you can't see which ones you have started watching and can be resumed.
But anyway, thats just a matter of opinion, actually I did no changes on the way how it's done. I just used the code that was already there. So when we like to change the behavior then we maybe should create a poll?
To be honest I don't understand atm how this could have worked different, even when going a while back through the svn logs.
No changes done in code snippet doesn't mean that it wouldn't break the existing functionality (like testers say that it has been working earlier). If you moved the code snippet (or method call) to a different place it could already trigger the bug testers are reporting.
I haven't looked at the original code (or patch) and I wont since I already know that the my videos plugin code is horrible and really messy (that was the original reason why I did propose the performance fixes to be done after 1.1.0 since they were considered risky).