mixing in surround, whether its LCRS or SDDS has its advantages and disadvantages. Stereo and Mono are cool for smaller venues, but if you've ever listened to a heavy center channel mix or a mono mix in a big threatre its' kind of distracting, as the sound is from a single point source right in the middle of the picture... and the picture is 50' wide. Mixing in stereo for the same type of venues can be distracting to the audience since there is only a phantom center and depending on where your sitting the sound, the dialogue can easily come from the wrong place. Mixing in LCRS gives you the center channel for anchoring the dialogue and a nice basic surround for general ambiences, but it to can have its issues if the mix is incorrectly done or if the encoding is incorrectly completed the steering can be off and/or something meant for the front can end up in the rears. Also if you want a dynamic surround filling the room you can end up with things coming for everywhere, like a single bird chirp, instead of from a point source. Mixing in 5.1 is tough, if only becuase of the multitude of options available to you and ensuring that your amazing 5.1 mix plays back in LCRS, LR, and Mono as well. SDDS can be even more fun to position and play with, but again you have to be sure it can be properly played back in all the other formats as well. And I personally HATE doing the bazillion splits to a bunch of 3348 tapes....
SO every format has its ups and downs... In the end , its more about what works best for the project and where the project will be seen and/or heard.
cheers
geo