I've been using those "aftermarket" knock off batteries for years - - and they work perfectly!
But go ahead and buy only official Nikon batteries, Nikon (and Canon) appreciate the consumer that only buys the genuine high margin manufacturer's products.
I agree to a point.
While I have never had a problem with any battery IN my camera. I have always go ahead and buy backup batteries which are from Nikon, going all the way back to my first D1.
I see no reason why I should try to save $40 on a battery with unknown origins to use in a body which costs upwards of $1500. ( or even $400 for that matter ).
But, here are my three real world reasons for that.
1. I bought a battery on eBay for a point and shoot I had. It shipped directly from Hong Kong and the third or fourth time I was charging it, it caught fire... thankfully I was home and in the room at the time because I smelled it and unplugged it. ( Only to be inquisitive and move it outdoors off an extension cord to see just how bad it would get... it didn't explode, but it did actually have a small flame which could have easily ignited something nearby in my room ). Certainly, having a house go on fire isn't worth the savings.
2. I bought a laptop battery on eBay and it arrived D.O.A. as did it's replacement... After shipping it back the third time I got one that worked well enough, but in that case it was worth the savings. ( the ebay battery was about a third of the $100 that HP wanted for a genuine battery.. in a 3 year old laptop that cost $400 new... it wasn't worth it ).
3. A coworker bought a Nikon D-40.. got an eBay battery and it leaked IN THE BODY.... it wasn't a giant mess.. but it was enough that it caused problems. Nikon refused to fix it ( it was grey market ) EVEN if she was going to pay out of pocket.
She found someone who was willing to work on it, but wouldn't guarantee that it would be fixed and she had to pay $90 to do so. As it turned out, he did fix it and it still works today... but it certainly wasn't worth the "savings" she thought she was getting.
This doesn't mean that manufacturers OE batteries are always perfect. As I recall a story in which defective batteries were showing up in some APPLE stuff as well as HP over the years that would just ignite... but the difference is, it usually gets recalled and most of the time you get your equipment replaced...
Would I use eBay batteries? Yes... but not in my good camera bodies and I have made sure I charge all those batteries in an area away from anything that can catch fire... just in case.