There is nothing wrong with getting more def than what is required for uncritable. However its not something to gem for if thats what you are wanting to do (unless you are trying to assemble an unhittable set).
Pros: Gain avoidance/mitigation
Cons: loose effective health
I am sure there are more pros and cons, but I am at work and dont really have the time to sit and think about all of them.
My suggestion is dont worry too much about geming for it but if you get an upgrade with more def on it dont be concerned that it has the extra, wont hurt anything. If you are wanting to assemble an unhittable set though I would suggest getting as much block gear as you can find and gem everything defense other than one or two gems to make your meta active.
http://www.tankspot.com/forums/f97/4...ar-glance.html (Here is a gear list with good suggestions on gear and gemming for each type of set)
*edited for link*