Tarkin is one of those characters I think really adds something. Much like Thrawn, he kind of shows how dangerous the Empire is even without the Sith but what he lacks in Thrawn's genius, he makes up for in sheer, terrifying amorality. He will use the Death Star at the first opportunity not because he has to but because he can. Because he simply does not care about loss of life...or life in general. And yet, he's not a Sith, he's not a big scary force-lightning dude, he's just a well-mannered, well-educated old British gentlemen who hasn't a shred of conscience in his body. And that's what makes him terrifying, how comparatively ordinary he is.