Its 5 years + since you posted Giles but only last Friday when I learnt how right you are when returning to collect my bike after having two tyres fitted I was informed that one of my valves was broken. Even though I insisted it was ok when I brought the bike in they denied damaging it so I am left with probably and perhaps someone can advise me on this a £200 bill for BMW to replace it.
Oh dear!!
They are very brittle. We get used to bending the conventional rubber valves to fit our airlines.
That's a bit naughty of the tyre fitter / dealer whatever if they did do it ....