Maybe because of my UNIX-influenced worldview which says that the things I'm using should be streamlined, do-one-thing-but-do-it-very-well, and should be "composable" as in ... via UNIX pipes, if we're talking about software utilities, and by putting them on the same network, if we're talking about computing devices.
Before the introduction of tablets, traditional computng devices performed the tasks now expected of tablets. They simply offered an alternative input method and a more convenient form factor.
We need to quit acting like convergence forces a computer to do something that its not well-suited for. Before the iPad, computers were used to consume content. In the past, we did have to make compromises, because the technology wasn't sufficiently mature. Now it is, and we don't have to compromise.
In my hypothetical scenario, the device is equally well suited to being used as a tablet or an ultrabook. Such a hypothetical device renders your argument moot, because your argument depdends on a hybrid device not being able to perform as well as a single purpose device. We are at the point where hybrids are a small fraction of a lb heavier than a tablet while in tablet mode, and a small fraction heavier than a Macbook Air while in laptop mode. This indicates that the technology is there, we simply need someone to come up with a good design.
Maybe because of my UNIX-influenced worldview which says that the things I'm using should be streamlined, do-one-thing-but-do-it-very-well, and should be "composable" as in ... via UNIX pipes, if we're talking about software utilities, and by putting them on the same network, if we're talking about computing devices.