the agenda of an art historian's specialist interest colours what others consider to be worthwhile.
Possibly although this would depend on the perspective. For example:
The study is based on available original documents - it lacks information that is not recorded.
The study is based on a collection - it lacks information that is not included in the collection.
The study is based on interviews of staff - the information is based on hearsay.
In each of those cases the historian will be making value judgements on the material and will, possibly, emphasis points that may be less important then they then become. In reality, combinations of these are brought into play.
Have art historians genuinely increased our knowledge of the subject, or merely directed people towards potential areas of the applied arts that become collectable, and then profitable?
If you allow the collector/dealer/researcher to be included in the scope of 'art historian' then there are many good examples of people leading the fashion in collecting. In the UK, Victor Arwas (Art Deco/Nouveau), Dan Klein (Italian Art Glass), Richard Dennis (British Art Pottery), Frank Andrews. (Ysart Glass)
In my own case I took a glass considered by only a few as worthy of attention, Monart, through publishing research in a club newsletter and later a book it became collectible. Other dealers had tried in the past to create a market for the glass but it had remained at the same price level for years 5-20 pounds apiece. By diligent marketing I created a visual impact at the centre of the collecting world in London. By sharing knowledge - publishing my own and others research in a dedicated collectors club. By publishing a quality book. However, instead of exploiting that advantage I then retired from dealing in the glass.
Now I am back to sharing the research that has happened since then. This brings up another issue in your debate that relates to the Stennett-Wilson/Midwinter point. Is the art historian responsible for this situation, should that historian have a duty to keep abreast of expanding knowledge on the subject of his published research? At this point is a 'conflict of interest' arising? The academic historian may have exhausted their interest in this area, whereas the collector historian is likely to continue their interest and bring later knowledge into a wider domain. But unless the latter sees that knowledge published it is likely to be lost. Yet another influential factor comes into play - willingness of a publisher to publish.
I can again use Ysart Glass as an example, the article on Paul Ysart had been written by, now Professor, Alison J Clarke whose interest in glass and Paul Ysart had been the subject of her thesis at university. She was happy to research and write the article and this was based on interviews with Paul Ysart, collectors and earlier research. The quality is good and has stood the test of time. However, Alison is a 'Design Historian' and her interests have moved on. There is more than one source for later research as well as some distortions circulating.
In 1989 I published an article on Nazeing Glass in my own newsletter and also in a Glass Association journal. I had not researched in any great depth and it was mostly based on an earlier article that had shrunk into obscurity. It was aimed at giving Ysart collectors a minimum of knowledge on a similar glass and not to encourage collecting of Nazeing glass. My article contained a contentious 'fact' from the original source, but nobody told me until a couple of years ago on this board. Also unknown to me, I was being pointed at as the cause of the disinformation. Once it came to my attention I changed it.
There is another aspect of the collecting philosophy that has been changing noticeably in recent years although the roots of the change go back to the 1920's. That is the original Connoisseur collector as opposed to what I call the aesthetic collector. I would imagine that many art historians still fall into the first camp and that is the relatively new discipline of design history that parallels the latter.
A good example of a connoisseur collector was Michael Parkington. He came from an upper middle class background and was a lawyer. For years he was a serious collector of early silver that got frustrated by being forced to keep his entire collection in a bank vault. He decided to dispose of it and collect English Glass. In so doing he became the most significant collector of English glass of all time. He started with the earliest English glass and steadily worked forward in time. He obtained significant amounts of original research material and studied the subject with academic fervour. Sadly none of this was published although I imagine his archive ended up at Broadfield House. When I first met Michael he had only just begun on twentieth century glass. We had long winding discussions in which he expressed his horror at collecting a glass that was of such poor quality but had to confess that my enthusiasm was contagious. He bought his first ever damaged piece of glass from me and four weeks later I had to take it back, it caused nightmares. But the chink in his connoisseurial nature had widened and before long he 'had to have' a piece of Monart of stunning beauty but covered in 'stress' cracks. He justified this at first by being fascinated at the damage but it was not the last damaged piece of Monart he bought. He had crossed the line and started to collect for aesthetic values. I think the truth was that till then his glass collecting had been deadly serious and through our meetings had warmed to the idea of letting 'his child' share his collecting interest.
He had another effect though, as the number one glass client of all the leading auction house and dealers he made them start to take Monart seriously. Prior to that Monart was almost exclusively sold by mid-range auction houses in mixed lots with other glass. The term Schneider like or Murano like being used to describe Monart. Today, you see endless pieces of glass being described as Monart or Monart-like, including Schneider and Murano!
To summarise this long winding story, yes the art historian has an impact but equally important is someone to sell the glass and someone to buy it. Each has a significant impact but none in isolation.