Its not that you 'have to root', its you WANT to root this device and unlock all its potential!
Currently, the only Market available to a stock Nook is the BN market...just the standard store. BN makes it 'difficult' for Nook users to get the Kindle app. Its not impossible, but if you are going down this road just to get the Kindle App, you might as well root the Nook.
Also, keep in mind that the Kindle App does not provide you the complete experience of the Amazon ecosystem. I'm not 100% sure what is missing but it is not 100% the same.