Telltale's cut-scenes are rendered on the fly using the game engine, aren't they? As soon as a cut scene is higher-res than the game itself, immersion is lost as you're obviously grabbed from the game and again when the cutscene is over and you return to the game.
Also, if your monitor needs to switch resolutions often there's a few-second blackout, which again, is very poor for immersion.
And as said above an on-the-fly rendered cutscene is a lot less file-size intensive than a hi-res movie file.
While I don't think that high-res cutscenes disrupt emersion, I also don't think that they're necessary either. Basically it's just expensive eye-candy, which is not what Telltale is all about. As for Final Fantasy, my favorite one, Final Fantasy Tactics, had only one high-res cutscene at the beginning, and it had the best plot I've ever seen in a game.
ok ok im sorry... just an idea...