Presumably you sought out and paid the premium for a highly capable employee because you wanted their above-average capabilities. If they're just going to meet the baseline and go home, the employer is wasting time and money relative to someone cheaper and easier to find.
If the company hired a highly capable employee by accident, or the employee deliberately took a low-salary low-expectation position to "coast" in, then maybe this line of reasoning holds up.
Unless their brain is only working 4-6 hours a day and after that they won't make much of a contribution.
I feel like my point is, if you're gonna want hourly work, pay for hourly work.
Take this scenario, I'm a highly productive dev. I work 5 hours a day and I'm burnt. But my manager makes my sit my butt in a seat for 8 hours. "Fine, whatever" I say.
Now crunch time comes and I'm pushing hard to 10 or more hours a day. I'm going to look back at the wasted time before in my seat with resentment. Because it's expected of me to give you more hours than my alotted 40 when the time comes, but when it's light I don't get to take that back.
If the company hired a highly capable employee by accident, or the employee deliberately took a low-salary low-expectation position to "coast" in, then maybe this line of reasoning holds up.