I think that could be the killer feature of this use that space for thin batteries maybe only 2500mah. i can carry 3-4 in my bag and have as much battery life as i care to carry around. and rather than push the charging to 30+ watts that turns my phone into a hotplate can recharge 3 batteries at once at 10w in same time. Bonus to apple on accessories sales
Problem is that magsafe means wireless charging which is highly unefficient. It's not that big of a deal for stationary applications, but for attaching a spare battery (which is itself limited by its capacity) you are probably wasting 30% of it on the overhead of the wireless power transfer.
