Manufacturers like to sow this doubt in the public's mind - no different to using branded versus own-brand washing powder - both do the job, one costs more than the other and at the end of the day your clothes are cleaner... there does exist this paranoia though that somehow a piece of not-<camera brand here> plastic with some not-<camera brand here> wires in it is somehow automatically inferior to a very similar piece made by <camera brand here>.
If it was a more complex piece of kit than a battery compartment, a couple of switches and a rotary dial then maybe there's a point - but a battery grip is close to as simple as it gets.