I'm not a music mod so it's from the perspective of a regular user: It's... too specialised to be useful. Sure, it makes sense to search for instruments but that's if you search for samples, not whole songs. There also is the logistic issue i.e. the mods: Do you really think they would make work to determine which instruments are used?
The only realistic exceptions I can think of are solos as those are defined by the instrument they're playing, not just their content.
Even then, the same instrument can be played in multiple ways, defined by the song itself. There is the key, there is the tempo, there is the time signature, etc.
Really, not all songs which use distortion guitars are fast paced rock or metal but may be very well used in a slow, calm song even if it's mostly in the background. Sometimes, the listed instruments are not even defining the song if they play more in the background than in the foreground.
There also is the case of ambiguity in (mostly) vanilla sampled songs: SMW's samples are very simplistic, in certain cases what I would say semi-chiptune basically so they're mostly defined by the ADSR. One example is the @4 which SMW uses as the chello but may also work as the better trumpet sample (most famously in Touhou songs as ZUN loves trumpets).
Occasionally, that makes defining the instrument a bit more difficult. One example I can think of is the mostly unsampled port of Romacing Saga 3 - The Last Battle
: The main melody is defined by sawtooth waves and organ but Maxo's version uses @1 (which comes pretty close to a sawtooth, in fact, but also sounds like strings) and @13 (which doesn't really sound like an organ).
Even chiptunes themselves have this issue what instrument plays as the samples they use can only
be improvised to sound like a real instrument. A simpler solution is to just mention that the song uses simples waves but then you might want to search for the theme rather than instruments again (not to mention we already have got a tag, retro
Okay, my layout looks ugly.