*snip*
You're looking at it the wrong way. if increasing the number of options wont change the number of builds in play then the fact that only a few builds see play in the meta is a problem and shouldn't be used as an argument against having more options. Plus, not everyone will be using the same 2-3 meta builds, most people just having fun enjoying the game won't. Not everyone is looking for the same thing in their builds, and not everyone just grabs the top build floating around the internet to play.
If there's no diversity in the meta then the problem is with the current meta/balance. With everyone being able to get 12 set pieces build diversity WOULD increase outside of these few problematic builds.