"[Belief in God] is therefore not essential to encouraging good ethical principles..."
That I agree with, but a belief in God does define what a lot of people's values are. It is also, of course, inherently divisive as long as there are people who do not believe in that same god. In the long run, ought we not encourage inclusivity amongst children? The Scout Promise, and its guiding equivalent, could just as effectively be replaced by:
"On my honour, I promise that I will do my best, to do my duty to others, to help other people, and to keep the Scout law."
From experience I'd expect that most children probably only say it because they have to anyway. That would mean that it doesn't matter overly if the 'God bit' is left in or taken out. The only people who care are the adults, and they aren't the ones saying it! But the version as I've proposed above has pretty much all it needs to have in it. The "Scout Law", or its guiding equivalent, is the substantial part of the code anyway: Trustworthiness, loyalty, consideration for others, courage, respect, self-respect, community, carefulness. There is no religion in that. Removing references to God, and even to the Queen, are not going to threaten the movement in any substantial way. That said, most scout groups and guide groups are based on church sites, so severing the link entirely would probably weaken the movement.
As I've said before, religion is a small part, and not a vital part, of the movement. In that case it should be possible and perhaps even desirable to take that part away in the interests of inclusivity. Who knows, it might even strengthen it and make the movement more attractive?