Mid vent, is a great gap filler but most say it's not necessary.
If you want to maximise the middle winds.
If you want to NOT have to swap out frames.
If you want a sweet looking triple stack.
I love mine, it sees more use comparatively than my Std and Vtd. I do have a PoloVTD which is like an XTRA though. If you have a PRO colour scheme chosen, get you mid as a Pro. You know you want to.....
Just one more, my Std, Mid and Vtd are all the same colours so when I upgrade it might be easier to sell as a package.