These 970 cards cost $370 in my country. So it is not profitable to make a rig with them if the Ethereum price drops.
No-one said you had to, no-one's trying to convince you either way. If it doesn't make sense, it doesn't make sense.
I need nVidia kit because of my work (HPC); virtually all GPGPU is CUDA-based, some is Intel Phi, but nothing of consequence uses AMD, or for that matter OpenCL (yet?). Doesn't make sense for me to buy any Radeon's even though they're "cheap", does it?
I haven't sold a single ETH yet, don't care where others think the price will go. No-one does, if anyone says so they're full of shit. I have my plan/goal, sticking with it, and if the whole altcoin world craters I could care less because I didn't spend money on hardware I can't use now and later, and I've only spent a small fraction of my electric bill on it.