I think that was a good example of keeping the audiences attention: that must be the most famous of all Shakespeare's lines in all his work, so the fact that there is a chance it wasn't said helps to differentiate the tv series, sustain some anticipation from the audience as they strain to hear the phrase, and underscore the uncertainty inherent in historical recording (due to apologists, renaissances and revisionists, or whatever). I thought that worked well, though I confess I was waiting to see if he spoke, and if he spoke in Italian (like some of the other key lines!).
Maybe, as the ScottishMartialArtist intimates, the problem is having to dumb down the show to the lowest common denominator.
Maybe Mel Gibson could re-do it, all in latin?