Some are clever and diligent -- their place is the General Staff.
The next lot are stupid and lazy -- they make up 90 percent of every army and are suited to routine duties.
Anyone who is both clever and lazy is qualified for the highest leadership duties, because he possesses the intellectual clarity and the composure necessary for difficult decisions.
One must beware of anyone who is stupid and diligent -- he must not be entrusted with any responsibility because he will always cause only mischief."
-Kurt von Hammerstein-Equord
Well, hard-working but not clever people will do useless work and create extra work overloading everyone else. BUT, this happens when they don't have good management and leadership above them, to prioritize and handle them bite-sized pieces, and to set their frame of mind to focus on "results in the bigger picture".
Probably putting "clever and lazy" and leadership roles is the cause for bad management and leadership which makes his "stupid and diligent" underperform in ways that drag everyone down.
Sayin' this as a "clever and lazy" person myself - I know that if I don't have someone I am responsible to (doesn't matter if it's a managers, partner, peer etc.) that is truly hard working and diligent is a recipe for total disaster. I'd rather be accountable to people way less creative, and even lower both IQ and EQ, as long as they come with a solid work ethic and focus to detail and they manage to drag me in this direction to! If the world would have only of people like myself in all leadership positions, nothing would ever get done, and the "diligent / hard-working" people will probably go crazy and either kill themselves or start wars.
I've always thought of "stupid" as different from "not smart". I can't fault people for not having the experience or knowledge that I or others have. I tend to think more of an "active stupid" as a lack of common sense, and making actually poor choices as opposed not making good ones.
To me, it's more the difference of "My laptop is overheating and I can't figure out why" versus "My laptop is overheating so I poured water on it to cool it off."
Not smart is not intuitively knowing what to do in a given situation - this can be learned away - it won't give you the intuitive answer, but at least you'll know not to do something in a given circumstance.
I like the definition of stupid vs. intelligent from there - intelligent person consistently makes win-win decisions. Stupid person consistently makes lose-lose decisions, inflicting loss on other people for no gain for themselves.
People whose work involves mainly analytical thinking tasks are often mistakenly perceived by public society as lazy while in fact they have always been working all this time, probably harder than most. It's only because the nature of their work is not physically visible to the naked eyes.
Quality decision can only be made after it has been given sufficient time to carefully analyze all the information and its available options. This can only be achieved successfully when people have plenty of quiet moments alone to really think about the specific problem. Those who are always busy doing something all the time rarely have this opportunity, they are always in a rush and don't have the habit to utilize the process or pattern required for analytical thinking. Just like how everything in life is a trade off, if you don't dedicate adequate time for thinking tasks, you cannot expect to formulate quality decisions.
This is the difference between me and my mom. She'll put off something for ages because she could theoretically do it herself, but I'll just hire someone else to do it after calculating the value of my time/opportunity cost vs hiring someone.