Old argument but the keyboards were designed with Stereo sounds and effects so I think stereo is the way they are meant to be heard.
Its not always possible or practical in some situations but when I can I always try to go stereo. Depth , spaciousness, and placement of sounds in the listening field are suble but very important qualities to the overall effect.

Come to think of it, not only keyboards, but also guitar effects. I don't play the guitar anymore, when I had purchased a Korg guitar effects processor I noticed that just about all the patches were in stereo.