Agree to an extent that Halo 5 should be fresh in regards to its music but I also think it will be good for the fans if 343 pays homage to the original theme or at least the choir part chord. Its the most iconic theme in gaming history and I'm sure Neil could work wonders if he used a bit more "iconic" themes (mombasa suite for e.g.) and darkened the feel the way he did with Halo 4.
In Halo 4 I only ever recall hearing the Halo theme once during the cutscene - and during those few seconds I was absolutely blown away - only because the whole game was missing that key element. After my first play-through I did not even realise there was a soundtrack because it was not memorable at all for me. Until I YouTube'd it and heard how well of an OST it actually was - I then went back and heard the pieces within context and it was great!
Marty and Neil working on the same project with the old and new theme for Halo 5 can only benefit the game itself. I see nothing wrong with going back to the old theme to pay homage to the beautiful music Halo has to offer.