Another problem is that it's a matter of framing, which is subjective. One guy, if asked to describe what one thing he did this year, might say, "One thing? Oh, god, I did zero things, because I was too busy doing a million things. I spent the first quarter putting out fires while we were scaling up the WhizBar, then I spent a few months looking over the call center's shoulder trying to figure out what they were doing to kill the DoodleBlat after we rolled out the Bilbo feature to production. At the same time we were debugging that thing that made flames shoot out of the Bilbo every Thursday night in the Central Time Zone during Daylight Savings Time. Then I was on that planning committee helping do traffic projections. And of course Q3 was when we did all the work to make the AcmeCorp deal actually work. Ask me about Q4 when it's over because frankly right now I don't even know what I'm doing this afternoon."
The same guy with a different manner of presenting himself might say, "I kept the servers alive all year, asshole. Now get out of my way or they're gonna die while I'm talking to you."
On the other hand, they are a single point of failure. When someone succeeds from outside in pushing the dictator out, chaos ensues. Some will thrive under the ensuing autonomy as they see room to advance their own agenda, but many of the most talented will move on if they perceive that the team is in a degraded state. Worst of all, being a trusted minion of the prior dictator leaves you with a suspect role when a new dictator grabs the throne. Experiencing this is somewhat painful and complicates your weighted decision heuristic for what to do with the rest of your life. Will things get better? How long do I wait? Could I possibly reinvent myself to the degree that e.g. patio11 was able? etc.
I've found that a number of my bosses have been quite understanding about this sort of thing. (I'm terribly lucky.) I have been able to pitch a new idea and get the nod to work on it. Granted, I've also never done the singular focus thing; I'm still required to pitch in with bug work and sometimes I'm the right person to ask for some random but important distraction.
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.