The last printing company I worked for still had a filmsetter, so we used a transmission densitometer regularly to calibrate the machine. I think it was the 361T table top version. They are becoming rare in prepress departments, but they were originally really expensive. I never had to purchase one, but I believe they were probably more than $3000 new. It appears mostly printing companies are selling these, with the intent of other printing companies buying them who still process film. For you and me, $500 is a lot for an old densitometer. For a prepress department with a budget, $500 is a drop in the bucket compared to the cost of software, RIPS, servers, computers, proofers etc. I just spend $10K to upgrade our proofing RIP!
Like Chan said, a lot of graphic arts/printing companies are getting rid of filmsetters and replacing them with direct to plate machines. Screen printing is one of the only industries that I can think of that would need film. Some pressrooms still use reflective densitometers, but they are being replaced by Spectrophotometers, which are more accurate in measuring density in CMYK. You wouldn't want to use a densitometer used in the pressroom for a negative though because they use reflected light, not transmitted light.