I don't even get the safety reasoning behind it, I think the only fatality that the halo would have prevented in recent years is Henry Surtees' death, but seriously, that was just so incredibly bad luck, you can't ever prevent it completely in motorsports. Even then, strengthening wheel tethers sounds like a much better solution than putting on a halo which brings its own security issues, as Rando and Tsubs mentioned already. It doesn't protect (or only up to 17%, according to FIA itself) from smaller flying debris either, so the incidents like Massa still could have happened, or could have been made even worse by deflecting the debris.
Weird solution, all in all.