Many users use the phrase "MIDI plugin" when they are really VST instruments. This is because VST doesn't actually have a MIDI plugin "type" but a VST instruments can process MIDI. SONAR supports MFX MIDI plugins that can be applied to a MIDI track, those are a Windows DirectX thing. I don't think very many DAW applications ever supported MFX plugins. Which is too bad really.
The MIDI-oriented plugins at Thepiz.org are very useful, but I wouldn't get worked up about naming issues. You just have to remember that VST plugins that process MIDI are instruments inside SONAR and can't be used like SONAR MFX MIDI plugins. Which can be applied to MIDI tracks as effects.