[e10jc]

In my past, I have both worked multiple jobs as a developer, and employed developers working multiple jobs. I have seen it bring a lot more benefits than drawbacks.

To say to someone, "you are not allowed to make money elsewhere", when they are able to fulfill their job obligations without issue, is rooted in jealousy, and creates a worse working environment for both employee and employer.

[s1artibartfast]

The problem obviously is when they can fill the duties

[AnIdiotOnTheNet]

If they can't fulfill their duties, you don't need "they had another full time job" as an excuse to fire them.