This is effectively the third try at making something like this work. Arguably the first "variable" font was Metafont which was limited by its programmatic interface which made most type designers reject it as a useful tool. The roughly contemporaneous development of PostScript and the subsequent opening up of the Type 1 format to non-Adobe font development was sufficient to end any possibility of MF being a viable type development system. Still, its fully programmable nature meant that it was possible to have parameterized typefaces and opened the possibility for something like this to appear elsewhere.
Take two was Adobe's Multiple Master extension to Type 1. Technically this is still used by Adobe Acrobat to handle fallbacks for missing fonts, but as an end-user technology it was a failure, largely because of difficulties in being able to use the interpolated fonts—few applications allowed direct selection of interpolated designs and users had to generate them in ATM instead.
So now we have the third attempt, with better application support, which has some hope of success. For text type users, the appeal is the ability to finally regain optical scaling for type designs. Traditionally, as a type design was created at a smaller size, it would become relatively darker with less contrast. Perhaps the most dramatic example of this in everyday use is to compare the spindly appearance of "fake" small caps with the more robust appearance of properly designed small caps. So this is one case where, yes, we really do want this ability.