I usually play in places where stereo is very feasible with no acoustic challenges. Someone posted here about a Harbinger 2.1 system on sale. It was dirt cheap at $350 but it came with cases, stands, and cables. I thought I was getting some "cheap enough to play in the rain" gear. However, at a gig I played my 88 key piano in stereo on the Harbinger, and I could not believe how smooth and good it sounded. It was very easy to set up, too Ever since then, I am sold on 2.1 systems both for mono and stereo. On most of these systems you can go either mono or stereo by the push of a button. One issue, though, it just didn't feel right when playing a $7,500 Roland digital accordion(I didn't pay that but other have done) through a $350 2.1 system. I didn't want to be penny wise but pound foolish. It was as if, "I got much more than what I paid for. To make it right, I bought a more expensive 2.1 system. There are a few good 2.1 systems out there; one made in England, but mine in Canada. Yorkville. It looks ugly in pics by the company (side looks), but it looks good in reality. I'm very happy with the sounds and its versatility.

I don't think one needs to run a digital piano in stereo though some Yamaha stereo piano patches can sound really flat when summed up in mono. However, multiple instruments from an backup or an arranger sure can take advantage stereo - for the acoustic distance and dimensions multiple instruments need. When I was testing my Space Station 3 for its effectiveness as a pseudo stereo system, it really gave a sense of big acoustic space and dimensions to all the instruments from vArranger. With the Roland Integra 7, you can position each channel or instrument not only from L to R but also from back to front as if you are dealing with instruments in a symphony. As Donny said, sometimes I want my audience to get the same "sonic treatment' that I get in my home studio if the environment allows it.