Given the choice of controlled deflation at 60mph or potential catastrophic failure I would go for the former and stuff the tyre fitter.
Also, anything which helps me prevent being stranded and having to change a tyre (or get anyone else to do it) on a hard shoulder is worth 20 quid a shot.
Fact is, it`s been proven to work, and anything I can do to increase my safety margin thats affordable I`ll give due consideration to.
I will accept there are lots of other products out there that work in a similar, but slightly different, way and lots which are a waste of space. It works for me, and to be honest, thats all that counts.