I disagree, but not heartily.
Of course something can hit it hard enough to push back and maybe even break it, but mine have taken plenty of hits over the years to the point I have to pull them back out and realign them on the clips, but with no damage to the headlight lens.
And there are some lights out there that are either hard to find and expensive, or like one of mine, made of unobtanium unfortunately.
So for me, the ten bucks spent (old times) on stone-guards was worth the possibility of saving the glass.
Had plenty of broken lamp experience before the guards. None after.
Not sure what a grille-guard will do for small stones, but for sure it's the only cure for a tree branch or deer-in-the-headlights (literally) issue.
I'm a fan of grille guards, but use stone guards in addition to, rather than instead of.
Basically for me then, even if the stone guards can't work every time (what does?) they were still worth the investment in insurance (which does not work every time either!) and that just-in-case occurrence.
And for most of us these days, they're just old-school-cool!
Paul