mark,
By mixer applications, I mean the the control app that comes with most soundcards. For example, RME has Totalmix, MOTU has Cuemix, Emu has patchmix, while Creamware's Scope has a virtual mixer as well (doesn't it's patch routing remind you of kxproject?).
Anyway, these mixers are not your average windows mixer. Aside from the usual pan and level controls, they usually have added features. For example, with Emu's Patchmix, the mixer has 'sends' that allow you to setup monitor routings, external effects options, auxiliary routing options and even use onboard DSP for effects with very minimal cpu load. I can control output levels individually (-10 or +4), set up S/PDIF or ADAT with my optical ports, select either S/PDIF or AES/EBU with my coax digital, and set up sync options among other things. Try to find those in a standard windows mixer.