Yes, it's that time of year again! Earlier this week was a good example when the 'O dark hundred' weather in central England became suddenly worse than the forecast. My night flight (transit, pickup the owner, transit to drop him off at a private HLS, transit home) was theoretically achievable without refuelling but my 'hunch' was to top it up to the gunwhales with expensive at EGLW (on first arrival at the aircraft in receding daylight the aircraft was wringing wet with condensation).
On my return to my base airport, the next airliner down the ILS went around... The nearest airfield to the drop-off HLS suddenly went from 8 kms to "2000m, shallow fog" as I listened to the ATIS on the way past; I had noticed ground lighting disappearing all around my track so the last leg home was flown "fully IFR" to the ILS. The "home" ATIS changed at least 2 times (conditions rapidly decreasing) during the 25 minute leg home.
My own interpretation:
If the planned destination becomes unsuitable for weather reasons after the aircraft lifts off, it could no longer be completed in "visual contact flight" so the flight must then be conducted under IFR in the normal "full" meaning of the expression.
In this case the options are to fly an instrument approach at the original destination, if available, or divert, possibly back to the point of departure.
You must be able to complete said approach, either at the planned destination, or at an alternate and should carry sufficient fuel for this eventuality. Once you have lost the "Visual contact" option, the flight cannot be completed as planned, so unless "full" IFR reserves were taken in the first place (including holding fuel at the alternate).....you are in an awkward situation.
The grey area to which I think you allude: Must you carry full IFR fuel in the first place, irrespective of the forecast, or can you carry "Visual contact" fuel to the planned destination with full IFR reserve only to the alternate?
I think the captain must be allowed to elect to do so, at his discretion even if it means reducing the passenger load. This means a good captaincy call before departure, having looked at the forecast and looked a little beyond it.... as often happens, it's the pilots' licences on the line to allow the operator to do the job in the first place.
It sounds like the Ops Manual ought to be more clear in this respect.