I do stitch in the ditch, quite often, and then I sometimes add free motion on top, sometimes not. I would like to develop better skills at FM, so I am working at it, but I won't give up SID even then.
SID is not easy. It's actually hard to stay in the ditch (not that I do stay in it completely). It's very time consuming, but I like the effect of outlining the patches, making the individual fabric pieces pop up and pucker after washing. SID doesn't interfere with the pattern of the piecing.
I am finishing up the SID on a log cabin with 1" strips, so after this process the layers will be well stuck together. I haven't decided whether to FM on top - but if I do, nothing will be shifting around so FM will be smoother.
So that is why I SID. I think there will always be a solid place for SID in quilting. I think that you don't see it often on quilts done by longarmers, because it's hard and time consuming for them, and they don't have the same need to stabilize the fabric layers, not because it's unattractive.
So, SID'ers of the world, unite!