I do agree that corporate multitasking often creates mediocrity-- that's painfully obvious-- but you can't actually get singular focus from people unless you give them autonomy. Otherwise, they're already serving 2 masters (their boss's assignments, and their long-term career interests which include getting out from under that boss's thumb) from the gate. So, "one thing only" really only works in an open-allocation environment, in which case you are by definition giving people the authority to decide for themselves if they prefer to take a one-thing-only laser-focus or a more multi-pronged approach. Which makes a hell of a lot of sense, actually, because everyone's different.
The problem with corporate multitasking is that it comes out of a lack of autonomy-- from requirements that come in from all sides and quickly leave the worker overwhelmed not necessarily with the amount of work (although that can be a problem, too) but with the total lack of coherence.