ND-2500A, at least on the popular earlier version of the 1.06 firmware, doesn't support CD-MRW or DVD+MRW to my knowledge. I'm pretty sure there'd be howls of protest if it was in the now-superseded 1.05 firmware then removed in 1.06.
That said, the entry is curious. Maybe Mount Rainier support was pulled for some reason before the final 1.05 firmware shipped.
I struggle to see the big advantage of DVD+MRW over DVD+RW. Background formatting of DVD+RW is supported anyway; certainly the latest versions of InCD support it. Further, you don't need special drivers to read a DVD+RW that was background formatted in a non Mount Rainier drive, whereas you do with Mount Rainier discs in a non Mount Rainier drive.
Of course, I'd prefer if it my ND-2500A did support Mount Rainier, but it seems it's not a feature the marketplace demands, especially as so many of the best-selling drives don't have it. DVD-RAM support falls into the same category - particularly DVD-RAM write support.