This isn't my field, but I'll try to frame the question more precisely. For any given wavelength in the plot, there's a magnitude representing its intensity. That magnitude becomes meaningful when viewed in relation to the magnitudes of the other wavelengths in the same plot. In other words, the shape of the curve is meaningful.
So, in general, if you were to take the plot of a spectrum as observed and reported by the PL Desktop Spectrometer and overlay it against the plot produced by whatever the gold standard spectrometer is, how closely would they match? I don't know how that's typically expressed in this field, but I could imagine it might be through some kind of r-squared formula of how closely the shape of the curves fit one another, or maybe it's expressed by how much difference there is when comparing the relative magnitudes (as reported by the two spectrometers) for each wavelength of light in the spectrum that's being measured, or some other way entirely. If I knew what the figure of merit was, I would have just asked for that when I first posted this question.
Anyhow, that's the gist of what I'm trying to ask. I'm not saying the PL Desktop Spectrometer would have to be as accurate as the gold standard to be useful, but just how useful it is may hinge, at least in part, on how much of a difference there might be.
For instance, if I had pointed the PL Desktop Spectrometer at the same patch of grass at the same time as whatever spectrometer was used to generate the responses curve for lawn grass shown http://publiclab.org/notes/nedhorning/11-01-2013/why-a-red-filter-should-work-well-for-ndvi , would the curve have looked the same? If so, then I could use the PL Desktop to do the detection described there.
When I made the inquiry http://publiclab.org/notes/WhiteRabbit/07-27-2014/question-can-i-use-the-desktop-spectrometer-to-analyze-ordinary-reflected-light, the answer I got was that there may be differences. So, I'm asking about accuracy in order to better understand whether those differences are big enough to matter. The specs at http://publiclab.org/wiki/spectrometer#Make+a+spectrometer don't seem to address this topic. Resolution is different than accuracy.