So this is gonna be a short post, but may I ask... why?
Everything else with offensive ratings has 2 offensive slots, only this doesn't:
1 offensive and 1 defensive. If I want to use a defensive slot then I use it along with a piece that also has defense, and even then - I simply don't need it due to how many variations of gear there are. Currently DPS want all their gear to be as much into offensive as possible, this knocks off over 4k power from a DPS to add most likely defense which can be filled from another sources...
Comments
I fully agree that having options is great. But this option is pretty useless, at the "endgame":
On a dps you cap every stat without a defensive runestone so you want as much offensive stuff(power) as you can get
On a tank its somewhat the same story, this gear isnt great for them because of the sheer amount of stats they need. So they will end up using double stat gear in order to cap everything. And again, this makes this comp gear obsolete.
And healers...well they dont want CA do they?
Defense is easy to cap as DPS, as tank you have a very wide range of gear to choose from... so overall defensive slot is more detrimental than it is helpful as of now since it is a 4k power (or an offensive stat in case a tank really needs it) loss.
This is part of the fundamental design of the system to provide a good mix of offensive and defensive slots.
I think it is more or less only for the rings and comp gear you can choose slot types.