I think the point of contention is the necessity of tablets. As ultrabooks get lighter and more power efficient they muscle their way into the 3rd device slot that tablets are meat to fill, all the while holding on to the 2nd and even 1st device slots.

I can identify with this. Our family got an iPad, and while the kids like using it, I much prefer my MBP for basically everything: reading HN, blogs, news, watching videos. The larger screen, better viewing angles, and keyboard just makes it superior. I'm not sad we got it, but I don't personally find it that useful. The biggest thing it made me realize is that it'd be really nice to have a laptop with a good touchscreen. I've been thinking about the Transformer, but I do wish that docked it'd behave more like a laptop-ish machine and less of a mobile-ish one.

I don't think that really works, though. It certainly doesn't for me. Much of the time, I like the lighter, smaller tablet form factor (even compared to the several netbooks I've owned). I definitely like being able to flip between horizontal and vertical screen aspects on a whim. Other times, I really need to have a keyboard, but I do have to increase weight and bulk for that. Therefore, my ideal would be a single device that does both without having to deal with syncing data between two devices. An "ultrabook" cannot do that, by definition; if it could, it would be a tablet with a dock. It's therefore not true that an ultrabook can fulfill the third-device role. Its far more likely that tablets will evolve to become adequate second (i.e. near-desktop) machines while retaining their third-machine attributes, but it's flatly impossible for ultrabooks to become adequate third machines. For many people it's not the tablet but the monolithic laptop that is likely to become superfluous in a two-machine instead of three-machine world.