Nikon d90 3rd party battery grip

Okay so I watned to buy a 3rd party battery grip off ebay. It was like 35$. My dad didn't let me buy it because he said that this is a 3rd party battery grip and it won't send the appropirate amount of energy from the battery to the camera. And that over time it will screw up the camera system and that nikon won't give me warranty coverage over it because the problem was from a 3rd party user. If nikon figures this out, will they actualy not give me the warranty coverage?

I personally avoid 3rd party grips and batteries. If they screw up your camera, the neither the manufacturer of the grip/battery nor the manufacturer of the camera will cover the replacement/repair cost of your camera. There's a reason the knock-off costs only $35.

Third party camera grips are usually pretty low quality, and often will not have the functions the factory grip has.

Why do you need a battery grip anyway?

I have a Nikon MB-D80 grip for my D90, but I only use it when I'm using my Nikon 80-200mm f/2.8 lens - which is very large and very heavy. I find that the grip helps balance the camera with the lens as it acts as a counter balance.

But when I'm not using that lens, I leave the grip off as it just makes the camera heavier and bulkier.

Unless you are using such a heavy lens, there's no reason to have a battery grip.