It's almost as good as an Aussie accent.
But if they want to walk around without body armor and kevlar, that's an unneccesary risk when compared with any possible gain and it's just plain stupid. If people want to kill a Brit or US soldier, they're going to try to do it whether or not the soldier wears body armor- although it'll be a good deal easier without. (I prefer targets which are vulnerable and I am sure most terrorists do too, which is why they like airliners, pizza parlors, and beating up on civilians.) Being vulnerable is not going to win anyone's heart - being deadly will at least get their respect. Being respected beats being loved any day.
The SAS is respected but not because of the way they dress.