True, but as you note yourself, there are lots of uses for home networks beyond just internet distribution. My wife is crazy for her camera and often has need to move hundreds of high resolution images from one device to another. A faster network would reduce the amount of time it takes to do that, since N can't keep up with even a single laptop hard drive. Backups are another task that would just take less time for a faster network. It's not necessary but it is convenient.
You don't need an SSD RAID to see the improvement. I have a simple centralized storage system hanging off a mini and file access is MUCH faster with a cable than over the air via N. I'm hoping that AC will be fast enough to eliminate the difference.
The resolution and frame rate are not the only variables. Image quality is obviously affected by bit rate, so higher quality renders can be much more demanding of bandwidth. If I'm editing a video and want to stream what I've done so far to the big TV so the family can see it and make suggestions, that intermediate stage is going to consume WAAAY more bandwidth than a finished h.264 encoded movie. N can do it, but AC should do it with fewer lags, burps and waits.