Is it not reasonable to ask an aircraft that is in VMC to operate VFR if that is convenient to everybody else.
Not at all as flying IFR doesn't neccessarily mean flying IMC (eg inside Class A airspace).
When an aircraft "changes" from IFR to VFR, their IFR flight plan is cancelled in toto and they would then need to refile if they susequently required to fly IFR at a later stage, and it also limits the maximum service available to the aircraft to RIS.
Again most company SOP's require aircraft to fly IFR to afford the the extra protection that somes with those flight rules. In the approach phase, more flexibility can be achieved by flying a visual approach under IFR, where the pilot can manouevre more freely maintaining his own separation from terrain and conflicting traffic he is visual with.
ATC can say to a pilot "report if you wish to continue visually at any time" suggesting that the traffic situation and the wx would allow a visual approach, but a controller is not permitted to either tell a pilot, or suggest to a pilot to change flight rules.