I'm personally happy for him to be there, as are most of the lads I've asked on the issue. We have a history of Royalty leading the military into battle and serving alongside which (although not in the same capacity) is still being honoured today. Any serving member that complains that his presence makes them more vulnerable probably needs to do a massive career-check and re-consider their profession - since our job is essentially to guard the royal family and the people of this country; it's well within our remit.
Besides, he's sat on Bastion for the most part more as a morale boost than anything else. I recall seeing numerous celebrities and key public figures walking around the bazaars when I was out there, they're all value targets for the taliban not just him.