This week super super busy? Work 45 hours to help out.
Next week slow? Take off Friday at 1 pm and work a shorter week.
Such agreements can be good for both employeer and employee.
The only real "problem" is if there starts to be abuse by either side. If 50 or 60h is the expectation for every week, then you should know that signing up. 60h week standard is "fair" from a salary POV, but make sure the salary is more than the 40h a week job down the road.
They are all happy (and in some it is implicitly mandatory) for you to work greater than 40 hours.
Oh, and unless I'm taking an entire day off I've never used any leave time for something like a doctor's appointment at any employer. If I miss a few hours of work one day I might make a special effort to make it up or might not, depending on what's going on in the department and my perceived need of whether or not it's valuable to the team/project. I'm sure soon enough I'll get hit with some overtime for a big project or a late night support call anyways.
FWIW I acknowledge that overtime is part of the deal on occasion working in IT, but I'm not the type to grind away 50 hours every week forever. I might be able to sustain that for a week or two in an emergency, but it's not worth it. I talk about that part of the work culture during the interview process. I would avoid grind shops.
This means the employer gets less simple hours work from you for the same amount of money, but that's the price of overtime.
The idea of renting your life to someone else for sustenance is at an anthropological level odd, don't you think?
Also from a capital point of view. Why work harder? In fact why not work easier if you are paid by clock hours.
Return to who - your employer or 'the market'? If your employer fails to monetize the widget you've just made, are you happy to get $0?
It's easier to move risk onto labour if they labour has more direct control over the return, which is why sales people are more comfortable with direct commissions than engineers.
Because my consulting practice is based on operational improvements and not ownership of the end result, I don't work on a commission basis but on value-based pricing.
Anyway, I was also a stoned teenager in food service when I got a job at a pizza shop delivering pies. No hourly pay, just $20 a day plus a $2/$5/$10 delivery fee depending on how far you were from the shop. I kept all the tips and payday was 11:30 or so every night. I've never worked harder or felt better about work than I did coming home with my pockets full of money at the end of the day - it was clear that my showing up and busting my ass made me my paycheck. Ironically, this is called "getting paid under the table" and it's illegal.