- It makes it clear that neither politicians nor plutocrats are the "top" of society.
- It encourages long-termism at the pinnacle of government.
- It makes real democracy more likely.
- It allows national symolism to transcend party lines.
I admire and respect some of my friends who have gone into politics, and there are certainly politicians who do so out of a genuine sense of public service. Nevertheless no-one in their right mind would claim that politicians as a group represent the flower of the nation and all that is brightest, best and most admirable. Much of politics is a very grubby business and some at least go in to it with mixed motives to say the least. Keeping the very pinnacle of society free from politicians is highly desirable for that reason alone.
Politicians inevitably tend to think in terms of the next election (at least in a democracy) and this tends on average to be about 2 years away. But most serious social and economic problems take a decade or more to deal with. The Queen has been on the throne since 1952 and can reasonably hope that her grandson will be on the throne in 2052 - and maybe even 2082. She can thus bring a 130-year perspective when every week her Prime Minister meets her to discuss the affairs of state and listen to her advice and warnings.
As for real democracy, look at the 2010 Democracy Index. 26 out of 167 countries are "full democracies" and of those 26, 12 are "constitutional monarchies". There are only 24 constitutional monarchies in the list. By contrast there are 10 republics in the full democracies and 91/92 republics in the other category (the People's Repubilc of China is not listed as a republic, which seems perverse to say the least). Nevertheless we can say that constitutional monarchies are 5 times more likely to be full democracies than republics are.
Fourthly in the nature of things, politicians tend to be somewhat divisive figures, who very rarely have approval ratings much about 60% and often have strong political opponents. Constitutional monarchs tend to be "above" politics and although there are always republican intellectuals they tend to be in a very small minority, and respectful of the person of the monarch even if they (misguidedly) think the instiution should be abolished. It is right that politicians should be criticised and opposed - it's great that this does not get muddled with our Head of State.
Finally, it's noteworthy that 4 of the full democracies have the same monarch. God save the Queen!