I had an “over-employed” person on my team (who lied about it) and I can confirm what all others are saying about this guy: they start going AWOL, miss important discussions, miss deadlines, blame their colleagues (creating toxic culture), start doing shoddy work because they’re not thinking deeply through problems and also to keep expectations low, create busywork for others to take the pressure off themselves, use company resources and accounts for other projects (creating security issues, among others)… just to name a few reasons.
It’s not about possessiveness. Many co’s are glad to hire contractors, who don’t “belong” to them.