I don't really understand why the full legal advice is considered so important, but I suspect the fact that the Government is trying to withhold it suggests that there may be something there, although I'd be surprised if there were some kind of smoking gun. Or could it be a false trail to distract from something else?
Because the advice will contain a section called "how we could break the terms of the withdrawal agreement" or something similar - and I imagine the government doesn't want the routes that they have mapped out for screwing the EU made public.
But why wouldn't you hide it in the same document that you were sure would not be published, as it isn't normal practice to do so? That way, you would make it readily available to all those who 'need to know', but keep it away from prying eyes.
Because the advice will contain a section called "how we could break the terms of the withdrawal agreement" or something similar - and I imagine the government doesn't want the routes that they have mapped out for screwing the EU made public.