It seems to me that if a departing IFR aircraft elects to proceed VFR because clearance is not available due to other IFR traffic, then a certain train of circumstances come into play.
The departing pilot must make an assessment that VMC exists. He must be aware of the cloud base and the amount of cloud that will ensure he can maintain VMC. Having, in another life, seen countless incorrect VMC assessments by pilots, I cannot see an improvement in pilots abilities to do this, especially if there is some pressure to go.
If conditions are marginal VMC, then the departing aircraft may have to divert around cloud and may not be on course within five miles. This introduces possible LSALT problems, traffic assessment problems, and subsequent clearance problems. (Not on designated route for lateral separation, perhaps)
The ability to see and avoid becomes problematic, and if the vis. is not so good, but still VMC, then people start to get a bit edgy. Particularly the other IFR pilot who would be wishing the other chap had stayed on the ground. Straight away, up goes the frequency congestion, and the workload.
This probably happens now in G, but at least the controller is not part of the separation link, and pilots may consider TCAS enough for their segregation.
I'd be happy to stand corrected if things have changed in more recent times.