There are two risks for this policy. The first one is "over-capitalization" where the use of a capital letter loses its emphasis because it happens too often. The major risk is that when a word starts with a capital letter in English, it really means something other than the simple semantic significance. For example, "Democrat" versus "democrat." I can see the capital for "Sovereign" since it is a reference for God, but to capitalize "Able" is a bit too much IMHO as it makes it lose a bit of its descriptive significance as an adjective which I see as the main significance here.
I see your point, but I find that "able Sovereign" looks strange. I do get the point about the adjective; some sovereigns are unable, such as the case with parliamentary kingdoms.