Yes, you could run the average (not for the GS-R) plug, and your car will still run OK. This is by no means a good thing. The previous owner of my car had the plugs for a B18 in my car. It ran OK, but runs much better with the correct plugs. The person he bought the plugs from probably didn’t know the difference. I ran into the same ordeal at the store – I asked for the GS-R plugs, and was assured, and given the plugs for the B18. So, 36 dollars later (almost 9 a piece for the NGKs), the car is running well.
I am sure that there are other plugs out there that can replace the NGKs, although, I would be hesitant about using other brands. Honda didn’t choose the NGKs because they suck. Do a search (using the function in the top right hand corner of the screen) on spark plugs. You should come up with some people’s experiences with different plugs.