While I like the idea of vertical slabs, I have a bet that the current behavior is annoys most people who otherwise would like to use this mod. I can imagine a rotating tool or something that flips slabs to vertical, and rotate adjacent slabs automagically to the right direction. The current behavior needs very precise targeting and could be problem if you'd like to place down a huge amount of horizontal slabs.

Also current behavior breaks most building wands including XU and BBW ones.