Here's how I'm looking at it. Speed is always a plus, especially if you put a lot of things inside your iphone like I do. Sure the iphone is fast, but if you store lots of videos and music and take up the space, you start to feel the actual lag. A lot of people use their iphones, but many of them just use a few apps, nothing that really slows down the phone so to them, the iphone 5 is blazing fast.
Another reason why you would want a speed boost is to assure you that the iOS will be progressing. You're going to need more processing power if you want to add more things to iOS. If Apple doesn't substantially upgrade the CPU, it'll give you an idea that iOS won't be going anywhere for a while...
Not sure why having a lot of movies stored in ROM would effect the speeds of the CPU. Also I've seen several iPhone 5/Galaxy S4 comparison videos, most showing the iPhone 5 to be as fast and often faster in performing similar tasks. Many mobile manufactures understand the lure of high speed CPU to most tech junkies, while also understanding that in terms of actual performance most of that power goes unused.