The Daallo Airlines bomb was handed to the passenger by ground crew after security. Which highlights where the real weak spot is in airline security. You can have all the security theatre you want, but if you have a handful of people on the inside then it all counts for jack. The MetroJet bombing proved that.
That's precisely what the ban guards against that scanning doesn't.
If you ban laptops, anyone who has a laptop is up to no good. You see a laptop, you make an emergency landing and have the person arrested as they should not have it regardless. If you scan them, an unscrupulous member of staff can sneak one on unscanned and while you're faffing you've got a hole in the side of your plane.
That in my view is the whole point. At trustworthy airports, the same threat can be guarded against simply by upping the level of the check.
Logically you'd also ban them from the hold, but you can't kill business travel as airlines would go out of business.