Huh. I never really thought about it before. I agree that the example you gave would probably piss me off, but that's more about the ship aspect (unless, like elisi said, it was an AU Spuffy vid. But if they were trying to portray the canon relationship, then I think it's very disingenuous to steal an "I love you").

I could see where in other circumstances it wouldn't bother me - I probably wouldn't even notice unless it was something blatant or a memorable scene.