How about a three scan approach.
DMAX, but not specific to Silverfast and applies to general scanning tools. In this case I am using the Nikon Coolscan+Nikonscan and post process to demonstrate getting the max detail out of a scan.
I was curious if I could gain shadow detail without blowing out highlights. In this first example, I am scanning Velvia 100 with an Analog Gain of 0, +1 and +2.
Larger version -> http://www.fototime.com/893E0E37B4C4087/orig.jpg
As you can see, default exposure of '0' looks too dark, +1 is more appropriate and at +2 the horse looses texture.
I use HDR to merge the three exposure settings as one image. Floor texture from the shadows and horse texture is achieved and it has that HDR look/coloring
I also used ACDSEE shadows tool (or PS shadow tool) to bring out the shadow detail on the analog '0' and bring out some shadow detail without the HDR look. So in an otherwise deep shadow, you can bring out shadow detail achieved in the scan.
This next one is from Kodak Portra 400 which seems to have endless highlight range.
Full res -> http://www.fototime.com/DCE615918D77901/orig.jpg
Again, the single scan has achieved the full DMAX, but in order to show the shadow and highlight details, you will need to use post to bring those out if those are important.
I didn't really need the three scans . . . unless you want that HDR look.
Under discussion is the question of whether the scanning software allows the user to increase the actual exposure of the scan, and if that is a benefit. I would say it might be a useful feature for those who scan slide film, but probably not if scanning only negatives.
Les, what is "Analog Gain"? Is that the same thing as increasing the actual exposure by scanning the slide for a longer period of time or using a brighter light source? - or is it something else? In other words, if analog gain is able to amplify shadow detail that was captured by the scan, well and good. But if the shadow detail was not captured because the scan exposure was too low for the density of the shadow areas, then that would require a different solution.
Your scan of Portra negative film has both shadow and highlight detail, and you conclude, therefore, multiple exposures are not needed. That might be a resonable conclusion if scanning only negatives, but it does not prove there is no benefit to using multiple exposures whan scanning slide film. In my experience, no negative has ever exceded the range that can be captured by a single exposure scan with my old Minolta film scanner. But my slides sometimes have shadow detail which I can see when the slide is projected, but which my film scanner cannot capture.
I have had some success merging two different exposures of slide film taken with my digital camera, and the resulting image has more shadow detail and well preserved highlights, compared to what I can get from from a single exposure from my Minolta film scanner. If your scanner is able to capture shadow detail from dense slides in a single exposure, I am happy for you - but for others, merging multiple exposures might be something worth investigating.
Finally, whether or not a merged image has "the HDR look" can depend a lot on what software was used to merge the images, and the capabilities of the person using the software.
