It isn't just that though, it's the symbolism of that and the statesmanship of the role. There are plenty of things that the US president does as far as dinners, banquets, hosting foreign dignitaries, etc, that go a long way towards diplomatic relations. A lot of US money is spent for the same things. No 10 doesn't do this here, it's generally left to the monarchy with them inviting the government--and is frankly a greater honour anyway (I'd be much more excited about going to dinner at the Palace with the Queen than with just the PM). Sometimes tradition and symbolism goes a long way and holds a lot of weight, even if it doesn't appear to hold much political power. I don't think a nation should just forget its heritage and traditions just because it isn't as PC as it once was or may cost a bit at times.
(I swore I wasn't going to get into this--I HATE arguments about the monarchy, and usually avoid them--oh well, damage done ).
What I meant was that if I ever suggest that there is a possibility that royalty are unnecessary, the only argument that I've ever heard from British people is that they bring tourists to the UK.
Regarding the Queen's role as head of state - there's no reason that the UK can't have an elected head of state, like the US does.
Since the UK seems to have much more of a focus on social equality than the US (universal healthcare, paid maternity benefits, carer's allowances, etc.), I find it a bit strange that the UK still retains a monarchy.
I don't like discussing this very much, either, so I won't post about it anymore. I've had my say.