Separation is a philosophy - LMCE isn't a specialist in storage, backup, redundancy, etc. It can do it, but if you can place your (valuable) media on a device that is better suited for these characteristics rather than on your (non-valuable) LMCE device, then why not?
By "non-valuable", this is taken to mean that the LMCE core device does not contain any irreplaceable data. At best it is stateful only - so a little config to get back to where you were, and even that is reducing with some of jon's work. So if your core was to fall over in a screaming heap, your first reaction would not be "oh shit I have to spend 40 mins rebuilding my core" it will be "shit f*#@! d#@$%, arrgh all the media I have been building up over the last 10 years is lost for ever!" See the difference?
On occasion you will likely need to rebuild your core, and so not having media on it vastly simplifies and speeds this process - you just stick a DVD in, hit the reset button and you are on your way. With media on there you may spend hours getting it all off first for safety, and much longer when you suddenly relise that you haven't got enough storage elsewhere to keep it!
Realistically, a simple NAS device consumes only a fraction more power than a hard drive on its own - do you really not have another PC in the house? Either way, you have more flexibility in setting up and scaling up or out your storage, redundancy and backup architecture when using NAS(s) or other equivalent, dedicated technologies. When you reach the limit inside a PC (like LMCE) it starts to become inconvenient, awkward, expensive or impossible to scale up. You can easily buy NAS's that allow you to add lots of drives, or daisy chain, or simply plonk another standalone NAS on your network as an additional share... no limits. Also, hardware independence - get a second cheap NAS, and use software to mirror between them. No amount of RAID will save you from a fire, but this potentially could (either in a NAS or an actual house fire!)