The big problem with "one thing" is the extremely political matter of who gets to set a person's "one thing". Is it the employee? (Open allocation.) Or is it the manager? (Then it's dictatorial, and you'll have to offer 30-50% annual raises to keep people.)
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.
The big problem with "one thing" is the extremely political matter of who gets to set a person's "one thing". Is it the employee? (Open allocation.) Or is it the manager? (Then it's dictatorial, and you'll have to offer 30-50% annual raises to keep people.)
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."
You hit the nail on the head. "Total lack of coherence" is the best way to describe the vast majority of the large corporate environments I have worked in during my career as a software engineer. All this talk of focusing on "one thing" is just more noise drowned out by the conflicting, in many cases contradictory "one things" the average corporate drone will receive from their many layers of superiors on a regular basis. The one unmistakable conclusion that I have taken from this is that corporations do not scale effectively. Everyone can have their "one thing" when your business is small, but once you reach a critical threshold, everyone's "one thing" doesn't make any sense in the broader context - unless, as you state, the leadership is dictatorial. In which case, good luck retaining the best/brightest/most creative individuals that are lifeblood of any successful business. People are not robots; it takes a nuanced leadership style that understands both the business and the nature of how creative individuals work, to truly be a successful leader. But that's not something you can quote in a four paragraph puff piece blog post.
Dictators can work well when they are insanely talented, motivated, and tuned into their minions. Being a trusted minion of a powerful dictator has provided the most effective, fun, coordinated experiences of my career so far.
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 suspect an organic middle-ground is what most best-cases end up with. One where the manager has veto power, rather than dictatorial power: the employee finds something they feel is their "most valuable contribution" and the manager says, "Yes, that's good. Work on that" or "No, we don't need that" or "Maybe later, but your current focus is this other thing" or "Think about it more and figure out how line it up with our company vision better."
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.
Open allocation is a middle ground. It's not a free-for-all. People are still expected to lead or follow, and still held responsible for working toward the benefit of the group. What you don't have with OA are those imbecilic internal headcount limits and transfer blocks. You're getting rid of an often useless and sometimes extortionate layer of indirection.
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.