> If someone is underperforming, do what you will. But why would you go looking for it just because they're doing a second job?
I've done some freelance work while also having a full-time job. It's great at first, when you still think that you can give 100% to your dayjob and fit the freelance work into your free time on evenings and weekends. If you can find relaxed contract work that fits to your schedule without detracting from your dayjob, then by all means go for it.
The problem is that contract work is rarely perfectly asynchronous like that. It won't take long until the company has some questions about your work that require a phone call during daytime hours. Or they find a one-liner issue that they need fixed ASAP, not when you have free time. Or they say they would like to continue paying you for contract work, but only if you can start responding faster during the day and turning requests around more quickly.
All of these pressures start adding up, tempting you to sandbag your salaried dayjob so you can have more hours to rake in that incremental hourly contracting money. In my case, I chose to give up contracting to avoid any moral, ethical, or legal breaches with my day job. However, I've seen many junior employees sabotage their own career progress by spreading themselves too thin with salaried work and freelancing work. Something has to give.
Worse yet, I've seen at least one case where someone used their company laptop and company phone for the contracting work for clients. Per your employment agreement, that puts the ownership of the code into a gray area. You don't want to go there, but when the clients are hammering your e-mail and phone for an answer it's tempting to just give in and interleave that work with your dayjob. Bad idea.
Finally, there's a myth that employees have a finite amount of work each week. When we do sprint planning, we calibrate to velocity, based on how much work the employee got done last week. I rely on my employees to help inform me about reasonable estimates and to help judge task complexity. Side hustlers have a misaligned incentive to exaggerate complexity, sandbag on tasks, and otherwise manipulate the system to minimize their dayjob workload. Less dayjob work equals more time for freelance income.
Bottom line: Some people can pull it off in rare circumstances, but more often than not a side hustler on your team will drag everyone down.