husker
Is only having one set of monitors/speakers  the reason to use a dedicated audio interface for everything? 

That is a logistical reason... but (IMO) not the main reason.
If you have a high-end audio interface... and you want to reference a mix against a well-done professional mix;
It makes sense to monitor the reference mix thru the high-end audio interface... as it'll have better fidelity.
Better D/A, lower noise-floor, etc...
 

Thanks Jim and Abacab.  I'll swap this over tonight to use only my Focusrite.   It would be nice to save a bit of space.
 
Edit:  Thanks guys, this really works well.  Saves some space, and sounds better!

I have found that I just like listening to everything on my computer through my audio monitors.  Everything just seems to sound better!!!
 
If I go back to the internal audio, everything just sounds dull by comparison.
 
Music played back through YouTube, Pandora, Spotify, and Amazon, not to mention my MP-3 library, all sound much clearer through a high end interface and monitors.

The speakers are the difference and the weakest link in any audio system.  I have some relatively high end Klipsch 2.1 computer speakers.  They sound great for PC music with my Realtek Audio.  My JBL LSR 308 studio monitors are great too, but with the Klipsch I don't need to use my recording interface and studio monitors for PC audio.  I like to use the PC audio to check mixes, and it gives a perspective on how the mix will sound (I also check my mixes on a car sound system and a boom box).