the nomenclature is pretty much meaningless
I beg to differ. Microsoft defined an "ultra-low-cost personal computer" (ULCPC) in its description of which devices were eligible for extended availability of Windows XP and for Windows 7 Starter. This definition, which covered screen size, resolution, RAM, and hard disk storage, ended up becoming the de facto definition of netbook hardware. Ultrabook, on the other hand, is a brand based on a spec set by Intel.
usually with slightly large screens
And there's the rub. Netbooks such as my Dell Inspiron mini 1012 came with 10.1" screens, but Ultrabook laptops are closer to 12". That can add up when you're trying to use a laptop while riding a full bus, and you'll end up needing to carry it in a larger, more obvious (to thieves) bag. That and Ultrabook laptops are far more expensive than your typical Atom netbook was, without an extended warranty to match.