I read the word 'stupid' as relating to the fact that lower speed limits were in force in spite of there being no obvious reason for it.
I don't think the OP objects to variable speed limits as such, just to them being applied in a seemingly random manner.
Obviously he was not suggesting that someone is having a laugh, just that the system does not work as it should, hence it was 'stupid'.
The word 'obvious' is key.
My perception is that knighterrant was trying to counter this by correctly stating that the reasons for the imposition of a speed limit may not be evident to drivers.
In regard to incidents, as opposed to congestion caused by volume, although my direct experience of operating such warnings is 20 years out of date, "in my day" speed limits would be indicated on a matrix when a member of the public reported an incident. A patrol would be dispatched and confirmation passed to the control room, or amendments made as necessary. Considering that many motorists on motorways don't know whether they are punched, bored, or countersunk, and haven't a clue where they are, or even which motorway they are on and in which direction they are travelling, it could be found that the signs which had been set were erroneous. (How many people don't know of the existence and purpose of marker posts?)
Although signs would be set as a precaution when a report was received - better to have them when they are not needed than vice versa - they would never be lifted until sanctioned by a patrol, by naked eye on the spot, so to speak, so if no patrol was immediately available the signs could be on 'unnecessarily' for a period of time.
The latter could be an instance of why variable speed limits remain on when 'not necessary', but as previously suggested, it could also be that they are on because they successfully efficiently regulate traffic.
Departing slightly from the strict point of the thread, my view of advisory speed limits on matrix, as opposed to variable limits which are enforceable, is that although many people ignore the advisory, they do at least alert them to the fact that there may be something ahead to be prepared for.
The ignorant often over simplify these issues and are totally unrealistic in their expectations.
Has anyone on here ever considered that 'speeders' cause congestion?