Same here... why would you want to use a regular LiPo charger?
The only semi-valid argument I can see is speed: the default P3S charger is rated only at 57W, which - at 17.5V - means just over 3A. With P3 batteries at 4480mAh, that's less than 0.7C charging. DJI sells a 100W charger, which means 5.7A and about 1.2C: not great (even the cheapest HobbyKing pack can take 2C, nowadays), but definitely an improvement.
And with P3 batteries in the $100 range, do we really want to push them beyond that?
On the negative, since you have no access to the balance points of the 4S pack, your LiPo charger would not be able to balance the cells.
Is the charger built into the P3 batteries known for doing a poor job in this respect? Has anyone opened a battery and monitored voltage/amps across the 4 cells to see what type of charging cycle it goes through?