Since the raw data from two separate cameras has to be processed in near-real-time to produce the final image. None of your previous experience of "better cameras" on iPhones measures up to this task.Since when did having a better camera require an extra gigabyte of RAM? That's a ridiculous excuse for having more RAM.
Having more RAM is great for keeping multiple apps open so they don't have to reload each time you switch back to them. I'd love to have 3GB on a smaller iPhone than the plus.
We know that Apple doesn't frivolously add extra RAM to its devices, so my guess (assuming this rumor was true) would be that two GB was not enough. It may not need the whole extra GB, but a smaller increase, like 2.5 GB, might be more expensive than 3 GB.