"Is there any reason why we would buy 4GB memory in 270X? considering that it is not a high end GPU "
"No, you're absolutely right. You'd never use a midrange card like the 270X in a situation where you'd really need more than 2 GB of RAM and its probably not a very smart purchase for an informed customer."
Actually, an "informed customer" would indeed know a situation where more than 2GB would be needed. In a single card setup, yes, 4GB would not be utilized as the GPU just isn't fast enough to fill it. The purpose of cards such as this is for a cross fire setups and in that case would definitely benefit. In a dual card setup, the amount of memory from both cards do not add to each other - Even though 2x4GB video cards would have 8GB physically, effectively you still only have 4GB of VRAM total. This is because all graphic information is "mirrored" between the VRAM on the first and second video cards, and then each GPU renders alternate frames. So in a cross fire set up, really you would need to think of this more as a 2GB per GPU card since the other half of its 4GB VRAM also contains the information being rendered on the 2nd crossfire card.
When you see cards like this, or in the past the Asus Ares series with a ton of VRAM, they are niche cards for people looking to run dual/triple/quad SLI or Crossfire.