For some, yes, all other things being equal, a single device that does both is better -- fewer things to carry, no need to synchronize apps/content between the two, etc.

For some other folks, all other things being equal, the utility of having two devices outweighs advantages of having a converged device.

Not that things are likely to be equal in this generation; while lighter and more power-efficient than convertible devices based on Ivy Bridge, and a lot faster than the Atom-based ones, Haswell-based devices are still going to be bigger, heavier, and more expensive than most Arm-based (and the smaller Atom-based devices.) They're also going to be a heck of a lot faster.

Before we've got a true best-of-both-worlds device, we're starting to get devices converging from either side.

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As someone with a lot more use for an ultrabook/subnotebook than a larger tablet, something like the Lenovo S230u Twist or the Gigabyte mentioned here is ideal for me -- mostly a laptop, but usable as a slate when space is very tight or when I need to read/review documents in portrait mode. Someone who needed to use it more heavily in tablet mode would likely find either one quite heavy/bulky compared to a removable keyboard solution.