Honestly I don't think it would be a bad idea using the Williams theme in pieces. Keep the Zimmer score sure, but BvS certainly lacked the MOS theme.
Coming to think of it, I think it's important to keep using the Williams score as a tradition. It's part of pop-culture. It was used in Smallville. The theme was used in the animated series.
Danny Elfman's Batman theme was used again in other versions, though it hasn't been used in a while. It should stil be used. But notice because of how popular the theme was, every other media incarnation advocated to re-use Elfman's original score even in other takes on Batman because it was such a popular theme that it couldn't be topped.
But as far as Snyder making the decision goes, I've learned now that I can only expect to be disappointed when it comes to him. In BvS...no Superman speech at the court scene, and instead an explosion killing nearly a thousand people. He gave us a Lex Luthor who is a total goof and a joke, instead of a cool, tough badass Lex Luthor we had been waiting to see for years. And Superman dying in the end in such a rush. Instead of having an exciting action-adventure film, it wasn't. It was mostly depressing and dreadful.
So I doubt Snyder would even consider using William's theme. Snyder already thinks his Superman is more relevant. This dark film universe that takes itself way too seriously. I think he is going about it the wrong way. He seems to think anything with humor in it is irrelevant and would be cheesy and wouldn't work at all. I think he's trying to be too much like Nolan, yet what I think he doesn't realize is...Nolan's Batman movies had humor in it. And Nolan wasn't taking his movies so seriously to the point where it was too much, and that he couldn't use cliches or make it fun and exciting. It didn't need to be 100% realistic which is what I think Snyder is trying to do with these films.
I like Zimmer's MOS just fine, but I don't think it's genius nor do I think Zimmer is a genius. A term which is overused these days and misrepresented. I love his Batman theme and Inception just to name a few. For the most part though, I find much of his music to be repetitive and lack new rhythms. The MOS theme "What are you going to do when you're not saving the world?" doesn't last too long. It repeats the tune a few times. Compare Zimmer's Superman theme to Williams, much more 'musical' I might say.
Whether Snyder uses it or not is not my main point though. I disagree that Williams' theme is dated and wouldn't work in Snyder's version. If you're talking about editing, those fan trailers edited with John Williams Superman theme in the MOS trailers, it won't simply work like that, of course not. The music has to be written and composed to each scene accordingly.
That's how John Ottman scored Superman Returns. Adapting the original theme, but rewriting it to fit certain scenes.
EDIT: James Bond has kept the original theme for over 50 years. I think Superman should do the same. Snyder seems to think so much of what was done in past Superman incarnations is dated and can't be used e.g. as the Williams original theme being one of them. I'm wondering though.
Maybe Snyder hasn't read that many Superman comics after all? Or he hasn't seen enough of the movies or animated series to show how awesome Superman really is....Superman in Justice League the animated series, he commands, he leads the JL and he fights, and he smiles too. Why can't we have that too? Or take Superman III for example, yes it was cheesy and had some bad moments, but it had its great moments as well, Clark Kent facing off evil Superman, then Clark finishing the battle, and suiting up and cleaning up his mess, enter Williams music. It's awesome.
So yes, I think Snyder can use the theme and that it wouldn't actually be a bad idea at all. So far though, he seems to lack the spirit of Superman.
