If the controller has other traffic which is IFR - then they need to provide IFR separation for that aircraft from all other traffic, not just other IFR traffic. Therefore if the other traffic is VFR and thus able to vary altitude to maintain VMC - more, not less airspace is needed to do that. So, their job is easier in crowded airspace that includes IFR traffic if *everything* is IFR. That might for example include the CTA around a busy airport with arriving instrument traffic, even if conditions are perfect VMC.
If on the other hand everybody else is VFR and the airspace is fairly busy, then a single aircraft going IFR is a royal pain in the backside. Thus it's wise to do your best to understand the situation and try and do your best to make everybody else's life easy if you can.