IO 540's comments of IFR operations in UK vs USA is very relevant to the AOPA quote.
In the US there is a very clear line between IMC and VMC flying.
If it is not VMC (i.e. 3 miles and 500 feet below cloud - generally meaning at least a 1000 ft cloud base then you are IFR and you must be in receipt of an ATC clearance.
In the UK in you just become IFR (and still wind up flying with the VFR chart because you can't go airways and you don't have a clearance).
I think this leads some IR rated pilots in the States to sometimes push on in reduced conditions trying to avoid going into cloud and then they unexpectedly loose visual contact close to the ground and are a statistic.
I am not aware of any significant risk with current IR/IMC pilots being confronted with declining weather and making a reasoned decision to transition to IFR - The risk is trying to fly visually in declining visibility. In fact, as Fuji Abound has so well articulated, this transition from VFR to IFR is a common feature of UK IMC flying.