Buy one, if you want to look like a twonk!
Seriously, almost no additional protection, from a flimsy bit of screw-on plastic, that makes you look like a reject. You'd get more protection by growing a big moustache and gurning at the dirt.
Either wear a full-face of or don't. I'm assuming that if you think that a Met Parachute will save your mug in the Alps, then you haven't been yet. Oh yes, I'll bash my face into a granite mountain, just as long as you put that wafer thin bit of wobbly plastic in between my face and the scenery.
Just like there isn't 'one bike that does it all', there isn't one helmet that does it all.
Unless you don't have to be worried about losing your looks of course 😉