The reason I'd buy an aftermarket is extra mah. For example, on a P2V, the stock DJI is 5200 mAh. That's the highest you can get. But you used to be able to get an aftermarket with 6000 mah. So that would be a reason to buy aftermarket, provided your willing to deal with the risk of a drop. The aftermarket would have to have a solid and extensive track record.
To buy an unknown aftermarket with the same stock mAh instead of a DJI to save a few bucks to fly your thousand dollar AC doesn't sense at all.