Donny has a point about singing to a room, but not every room has the same requirements, and not every singer can fill the room the same way.
I sing in many laid back rooms with just the on-board speakers in the s950, and that's the main reason I have one. I have grown all too accustomed to having that "in my face" monitor, and I really miss it when it's gone. The natural acoustic sound of my voice can fill many rooms without amplification, so adding the extra sizzle of the speakers and effects is a nice blend for small rooms and background situations.

I also play my guitar AND my iPad thru those same speakers ... I need to hear the whole mix before I send it to whatever house system I use (the Bose Compact most times). I use the Bose T-1 Tone Match mixer to feed the Compact and the "aux out" feeds back INTO the Yamaha.
Here's the layout:
Ch1- AKG dual wireless mics (for guests) WMS40 Pro mini2
Ch2- My Line 6 modeling guitar, or new Roland G-5 modeling Strat
Ch3- iPad (DJ tracks and custom backing tracks I've made over the years)
Ch4- Yamaha s-950 (aux out is MUTED - very important to avoid a feedback loop from hell)
***Channels 1,2,&3 have a dedicated aux send that feeds directly into the Yamaha aux INPUT, and I have control over volume, EQ and effects from the Bose Mixer.
I keep the main volume of those 3 channels at "0" for most rooms because the combined output from the Yamaha is feeding the Bose Compact and it's already premixed. In unusually loud circumstances, where I need to pump the guitar or DJ tracks or the best man is speaking too softly during his toast - I simply turn UP the main feed to the Compact and the extra volume doesn't overdrive the on board Yamaha speakers. What I hear is always a nice blend, and what the crowd hears is what they NEED to hear.
It's a little confusing to anyone who hasn't used a submix before, but it's VERY versatile.

Anyone need advice on this type of hookup with your own gear ... please give me a shout and I'll talk you through it. Sometimes, having that extra control at my fingertips makes all the difference between sweetness and struggling. C'mon ... to be fair - Yamaha never intended these speakers to handle the load I give them, but with proper care - it's a great setup!