If you look at the world of business-targeted Windows 8 tablets, many of them have 128GB storage options...including the upcoming Surface Pro. The logic is clear, especially if you're in a business where you need to store tons of files locally and don't want to sync in the cloud. In that sense, a stepped-up storage option for the larger iPad makes obvious sense.
The real benefit of 128GB of storage, though, would be in the 11-inch MacBook Air. That $999 entry-level device is the only MacBook under $1,000. It's a wonderful, compact laptop. It's fast. But 64GB of onboard storage just doesn't cut it...especially when you can't swap in a standard SSD upgrade.
The 128GB iPad is a step-up model. It costs $100 more than the 64GB version ($799 for Wi-Fi, $929 with added LTE). But now that iPad's only $200 less than a MacBook. If I were to pick which one I'd prefer more storage space on, I'd pick the Air. Most other Windows laptops start with 128GB SSDs. It's time for the Air to follow suit, too.