Even if you don't ever mix the 9-5 time with the freelance time, (Which FYI is nearly impossible in practice.) you just aren't going to be at your best when you show up the next day to work your 9-5 8-hour day after having worked a full day and moonlighted for another 4-6 hours afterward the day before. You're lying to yourself if that's what you think. And if you're working weekends, you're going to get burned out in a way that only punishes your 9-5 employer for not giving you a higher workload. People get fired all the time when their weekend or after work binge drinking interferes with their work the next day, why should spending your discretionary time burning yourself out be any different?
Human beings can only do one thing in any moment. Worse, this truth has some applicability over longer time scales as well.
Human beings require substantial periods of recovery in order to perform optimally. Often, what you do during that recovery has dramatic implications on how well you're going to do when you get back at it. (Ever solve a difficult programming problem only after taking a break? You won't get those same eureka moments if on your breaks your loading entirely new problems into your brain all the time.)
You do the math.
On site standard days are 12 hours, crunch time 14's become normal. Depending on project this might be month on month off, 5 days on 2 days off, 8 + 6 days, sometimes every day 3 months straight, or more.
You do get used to it and can be overall productive for the hours - usually company is paying (big) by the hour so they wouldn't be paying if they are not getting.
Not everyone can do it, or wants to, but different people have different capabilities.
I've been doing it for decades.
If the employee is meeting expectations or exceeding expectations then it is ok?
But not all things are equal. Some might be less than "one", and not require a full 8-hours.
Now, I wish I could say I did something smart with that time, but it was a long time ago, so I mostly spent that time playing Starcraft or chatting on IRC :)
The ones who only have 1 job but spend some hours at the company recreation room and less than 40 hours on work should also be fired?
