Skeletons don't have their bow when spawned using /summon if one or more NBT Tag are specified and HandsItems is not one of them.

When executing "/summon Skeleton ~ ~ ~", the Skeleton has his bow.
But when one or more NBT Tag is specified and HandsItem is not one of them (example: "/summon Skeleton ~ ~ ~

{LeftHanded:1}

"), the Skeleton spawns without his bow. The same happens to Wither Skeletons.