The reason for the open bottom is due to the thin nature of the plastic used.
Check out the Kate Spade leather case. It covers the bottom just like the Apple leather case for the 5S.
It's the perfect leather case and is priced accordingly at $50.
Unfortunately it has a fatal flaw!
The cutout for the lightning charging connector is offset and you cannot charge the phone with the case on the phone!
I tried looking at them at different Best Buy locations and they are all the same.
A handy person with a Dremel could perhaps fix it but two things come to mind:
This is a premium priced product and one should NOT have to do this to get it function as designed.
And two, opening this cutout will make the plastic part near the edge of the screen dangerously thin and would probably break over a short time.
With an open bottom like Apple has, this issue does not exist. It's not as pretty and could possibly be less protected (however the geometry suggests a drop where the bottom would strike first should keep the edge of the phone off the floor). A rough tar and chip driveway, probably not! Then again drops from waist height to a tar and chip driveway are likely to crack the screen due to contact with a stone anyhow. (this applies to most cases including otter symmetry, speck, tech21, etc.)
Slim cases are just that. Nice to keep a device looking new (no scratches on the back from it sitting on an end table and spinning around when the cat jumps across it, for example). Small drops are usually no problem inside onto carpet/hardwood floors.
Guys working on construction sites, riding dirt bikes, etc. are going to want something far more rugged for sure!