This is where Apple controls them, tests them and has them available through the store. Just like an app. For what it's worth, I've run themes since my old 3GS and they don't 'break'. They replace all your images with new ones and if a new image isn't available (let's say) for a new app, the apps original icon stays in place. Most of the themes in the jailbreak community are rock solid either way.. Not a lot to worry about.
I really like the theme idea. I had some of that on my 3G when I jail broke it. I would jailbreak my iphone 5, but on my 3G it just slowed it down so much.