You're mostly barking up the right trees here - 'Idiot Boards' as they're sometimes affectionately known are usually put in place as spad mitigation attempts for multispad-ed signals, which of course can be for a variety of reasons. they're usually on open running lines where visibility is restricted or where there's a risk of driver inattention/distraction. There are a few dotted around the Manchester area where i drive, one approaching Dean Lane station on the down where there's a reduction in speed to 25mph over thorpes bridge junction, followed by a 60mph where inattention could lead to a driver opening up against the red, which has very restricted visibility under dean lane road bridge. There's another approaching Lostock Junction on the down where the signal has restricted visibility due to curvature of the line and foliage, the linespeed here is 75mph and it's a long signal section from the previous signal where trains would still be accelerating after leaving bolton. These boards are not infallible as has been unfortunately proven by a recent spad in the guide bridge area. I don't sign that area yet but from what i've heard the driver set off from a platform with a single yellow at the end, negotiated a crossover at 25mph, opened it right up and only saw the red when it was too late at about 50-odd mph, the train passing the signal by about 30 yards after the TPWS activation, this signal apparently has 3 countdown marker boards on approach.
It's a case of 'every little helps', obviously a driver should react to every cautionary signal in some way (this helps as a reminder - doing something different ie reducing from notch 7 to notch 6 when accelerating after passing a double yellow for example, or coasting, or braking, whatever's appropriate) but drivers are human beings and human beings are not robots - better drivers than me have had spads i'm sure and thinking that way and being professional will help me to (hopefully) avoid spad-traps. Route knowledge is absolutely invaluable. All drivers will be aware that signals exist, not every driver will be able to pin down the exact location of every stop signal because some signals just aren't very often at danger. We are all assessed regularly, some assessments involving route-conducting/map drawing and i can tell you that any driver who misses out a main-aspect running signal, especially a multi-spad, will fail assessments.