From what I could gather so far, the smart batteries are not much different from other smart core systems used in different appliances (like the ones from Lupine Lights for instance - top stuff btw). Basically, they control the charge and discharge parameters to keep the battery in a certain condition, avoiding deep discharges and overcharges. They must have some kind of management and measuring/reading systems as well, to interact with both the P3 energy control and the app display.
Charging and discharging must be controlled in accordance to battery/cell specs, also considering the whole energy demand parameters of the AC. So a system must be used either on the charger, battery or the AC to control the discharge, and in the battery or charger to control charge, to keep things right. Both are important, as both can damage the battery if done wrong. Those are rather simple stuff really.
DJI put the smartness (so to speak) in the battery, I'd guess for both economical, manufacturing and customer care reasons. Even though that means one smart core for each battery, it's more simple and easier (I'd add "safer" as well) to use, from the customer's side. And that seems to be one of main DJI's targets. But I doubt they have any sophisticated feature on them such as cell balancing capacity or stuff like that. Heck, that would mean serious electronics. I own a couple of really smart and advanced chargers, you need to access the individual cell taps and all to perform that... Anyway...
So, smart systems used in chargers can be more sophisticated and thus more expensive. And it'd be necessary a second smart system on the AC anyway, or else the pilots would have to time the flights more precisely and measure voltage right after landing to get accurate readings on capacity and state, just like we do with RC copters, planes, cars, etc. So having only one on the battery itself covers a lot of bases. But it's certainly more basic than one found in some chargers, that's for sure. Maybe that's why DJI recommends a "deep discharge" (8%), to sort of calibrate the smart system and get more accurate readings (percentage and voltage) after a few cycles.
That's all fine and dandy, hobbyists use a voltmeter and some simple math right after flight to get accurate charge/capacity of LiPos all the time. I still do with the DJI battery so I don't really see much of a point in performing the deep discharge. I'm yet to compare the readings on the app and battery to my "manual" readings but I suspect it must be within acceptable margin. if it doesn't, then I'll do the deep discharge and see what gives. I sure doesn't hurt much, if done every 20 or so cycles I guess it's fine.
Also, I'm pretty sure they wanted to keep everything into a "closed system", pretty much like Apple. That makes sense from the economical/business perspective (they sure make a heck of a lot of $ from batteries, the margin must be big on those); increases the overall quad safety (debatable, but likely); and allows them to keep the whole quad within desired parameters of operation, by avoiding "third party" solutions, mods, improvisations, etc.. And the overall P3 design gets cleaner and more streamlined as well.
Dealing with "dumb" LiPos is a nightmare, even experienced RC hobbyists screw a battery or two every once in a while. If you have a few dozen ones - and different ones too, laying around and being used all the time for different applications, which is quite common in RC... Well, it's something that I'd guess the majority of P3 customers would hate, it'd be a real turn down for many.
