If you're talking about working for yourself (that is, working for customers), then sure, that makes sense.
If you're working for someone else who is paying you specifically to work 40 hours, then there's no reason to work more than that unless you consider your time worthless.
I frequently put in extra time on projects without compensation. I care more about doing the job right than I do about squaring up a timesheet. It's not about my time being worthless, it's about my life being worthwhile.
If you equate worth with money, in that case you are correct.
Hopefully there is a lot more than money to your job or, no matter how many hours, you'll likely not enjoy that work week.
If you're working for someone else who is paying you specifically to work 40 hours, then there's no reason to work more than that unless you consider your time worthless.