I suspect the real reason for the wide V with no flat top is for optional use of a moon sight. Because the moon sight is so much bigger, the flat-topped version could leave the bead sicking out the top.

With ordinary U or V tangent sights, I was taught to align the top of the bead or post with the flat top of the rear sight. You can't do that with a wide pointy-top express sight, so most seem set up to require a 'fine bead' alignment. This works OK on the first shot or the range, but I've heard a couple of professional hunters say clients often shoot high on the second shot - maybe they forget to pull the bead right down into the V in the excitement, esp. when there's no low flat top to remind them to keep it down.

This is not a problem likely to occur with a peep sight, of course.