Justin, three points:
(1) To use Silverfast you need to buy the version that is specified for your scanner. Unlike Vuescan, Silverfast is customized to each scanner model and the price of software varies considerably from one scanner model to the next. I wouldn't want to speculate about why, but that is how it is. Once investing in something like Silverfast, I believe it is worthwhile buying Ai 6 Studio - this is the full-featured version.
(2) As for using a spectro to read colour and create profiles, recall that we're dealing in this case with colour negative film. The raw data the scanner reads is reversed colours. Hence you would need an accurate IT8 target made up of patches of reversed colours for profiling such a scanner to interpret and reverse negative film accurately. There would need to be a very large number of IT8 targets (Negafix alone has well over one hundred choices) to faithfully portray the reversal colours of each film, because the reversal colours differ amongst manufacturers, and within manufacturers each film type and film speed. I suppose firms have judged that the trouble and likely sales volume for each target just isn't worthwhile, hence the Negafix fix in Silverfast and a more limited similar option in Vuescan.
(3) Silverfast is a needlessly complexified piece of software with a horrible user interface, arcane nomenclature and very poor quality documentation. All that said, from a technical, results-oriented perspective it is still the best game in town for scanning negatives, their e-mail tech support is responsive and helpful and once mastered it is actually quite routine and easy to use.
