If everyone is spending 5 minutes working every 4 weeks... why would you hire extra people outside of contingency coverage?
If my workload is such that I can work four "full time" jobs and every one of my employers is happy with my work product, then that's nothing but great for everyone.
Again, high trust environment.
If they're performing okay during the ramp-up period, it stands to reason that they'd continue to do fine after the ramp-up period.
But again, if the employee isn't meeting standards, you fire them and find another. Turning the employment into a "jealous and paranoid lover" situation where despite the fact that the employee is meeting performance expectations, you're constantly questioning whether or not they're *GASP* doing work on the side for Another Company(!!!) does noone any favors.