I do find it strange that while we all agree that volume is the best measure , we ignore what it says about where we are in the freeze/melt cycle . Peak volume is probably 2 or 3 months away , at which time melt finally exceeds freeze .. Neither melt or freeze are seasonal in that both are always ongoing , unless we have passed the point where that is true , yet twice a year excitement builds . Are the Inuit and the polar bears awaiting the news with bated breath ? AS IF ! 
b.c.
Friends, Romans, Earthlings: As one who has disparaged Extent as an incomplete, superficial, and highly variable manifestation of Volume, I come here today not to bury Extent but to praise it.
Yes, we all agree that if the question is how much ASI remains etc., Volume is the best measure. But a major importance of ASI is its effect on Arcitc albedo and Earth system energy balance. (That whole bit about white polar caps acting as reflective mirrors, that are turning into dark absorptive open water.)
So while Extent has its problems (as all the metrics do), it is a way to track the albedo dimension of ASI influence on the climate system. And Extent has the advantage of being the easiest dimension to directly measure.
Technically, Area might be a better metric to use for that purpose. But Area is derived by multiplying average concentration within each grid cell x the Extent, so while Area is more closely associated with the objective (tracking albedo status), that extra layer of abstraction and estimation introduces more error. At least that's how I understand it. Someone can correct me if that's wrong.
Thickness is even more difficult to directly measure than Extent because while we can see the horizontal spread of the ice, it is a lot more difficult to see the vertical dimension. Thickness is also important on its own because ice of different thicknesses has different qualities with respect to melt rate and mobility.
Volume is a function of Area and Thickness, so suffers from the estimation error within each of its components. But if you want to know how much ice there is, Volume is the only measure that describes that.
The ease of access and direct measurability for each metric decreases with how comprehensive is the information it provides. It is kind of like renovating a house - you can have Speed, Quality, or Cheap. But you can't have all three at once, so you have to choose where you are willing to compromise.
Extent, simply being the outer edge of the ice (>15% concentration), is at first appearance an inadequate way to measure the status of ASI upon which the habitability of our planet depends (a realization which is relatively new to many of us, and which is still unappreciated by too many of us). But it works well enough, and thanks to JAXA, NSIDC, JCG and Gerontocrat we get daily updates that allow us to monitor the situation with day to day granularity.
Thankfully, we also get Area updates, and thanks to PIOMAS, HYCOM, CS2/SMOS, Wipneus etc. we get regular updates on Thickness and Volume as well. All of which is still amazing to me, having been born before the first grainy B&W picture of Earth from a distance was taken.
I am frequently struck by the irony that our technical brilliance and ability to monitor and understand our planet advances at a rapid pace, and in a very close race with our irresponsible stewardship and self-defeating carelessness to foul our own nest. I read shocking venal stupidity from the mouths of politicians on the same day I get to see stunning 4K video from Mars and read about advances in quantum manipulation that reach into the mystical realm. These are interesting but strange times. Let's hope the better half of our divided character wins the race. Turns out our Moms and Dads were right, braun and brains are important, but character matters even more.
