Speaking generally, 0.5 db difference in level of whatever is considered (by sound engineers) to be about the limit of what can be plainly heard by anyone; finer differences are also heard, of course, but require attentive listening and decent equipment (also ears vary). This means that someone with a reasonably good stereo system listening to a Pianoteq recording can very well hear/feel the lack of precision inherent in 1 db resolution; certainly I do. Ask any, say, mixing engineer if he would agree to use a console that does not allow less than 1 db adjustments; chances are he'll just laugh at this preposterous suggestion. Adjusting spectrum is quite analogous to mixing. 0.1 db increment is not very necessary by itself, I agree, but it allows 0.3 db or 0.7 db or whatever; and, occasionally, I actually have used 0.1 db increment (in my DAW).

   Voicing
      spectrum profile level of different harmonics

   Sound Recording
      level of microphones

   Volume

It is important that the finer resolution of Spectrum Profile and Volume be available in Note Edit as well as globally.

I do not use the Pianoteq Equalizer or the effects, but for people who do, 0.1 db resolution is probably desirable.

There are other db parameters (noises, for example), but it does not seem to me that finer resolution is necessary there.