MIDIs (by which you are most likely referring to .mid files), don't contain recorded audio, so I guess you could say they're not "sound files." MIDI files are like sheet music for your computer; they contain instructions that your computer uses to generate sounds/music through either a sampler or a synthesizer.
As for the general opinion of MIDIs? Not really sure. In the day and age of broadband internet, actual recorded music is much more accessible than it used to be, so looking for MIDIs of songs just to hear your favorite tunes doesn't really have the same impact anymore. Still, I believe artists still find the file format convenient for a variety of reasons (reference, collaboration, etc.).