Just because you can use synth well (which Davidge would, coming from where he does) doesn't mean you have musical talent. It just means you have sound design talent, and those two are two separate things.
I've had several people tell me Green and Blue was a masterpiece. It's not. It really, really isn't. Davidge doesn't seem to understand -- now I'm blank for the term. It has to do with doing things in sets. If you're in 4/4 time, you do things in set's of four. If you're in 3/4 time, you do things in sets of 3 or 6 (since 3/4 time is also technically 6/8 time as well). Green and Blue just kind of goes, and then goes, and then goes, and it's like, dude stop. Move on to the next part. Oh, ok, that part was pretty good now...you just left it hanging. Where were you going with that? It very much plays more like an improve piece. It's got a good theme and stuff, but he just didn't follow through with it at all. He doesn't seem to understand music and how structured good music is. It's like he just wrote what felt best at the time as per emotions and what was going on onscreen.
If you say he couldn't have a perfectly set score because he had to match what was going on between Chief and Cortana, then that is an issue with the directing. If you'll listen to a lot of OSTs by writers like Zimmer, Williams, etc., you'll notice that they still follow the rules and timing, etc. of good classical music, and yet still match up with what is going on on the screen. If Davidge could not have done this, then either he or the people directing the scene have some things they need to learn.