I bought my first sideby this year, purely for sentimental reasons. I've always wanted one since I was a boy, in 20 gauge. I'm not interested in waterfowling. Upland and turkeys for me. I have always been fairly utilitarian when it came to my shotguns. They were pretty much always an afterthought, but everyone needs one, right?
That being said, just a couple days ago I purchased a little used Beretta AL391 Teknys in 20g that had just come into my LGS about an hour before I got there. I have been wanting to get into clays, which will be a new venture for me.
But back to the original question: from a purely practical standpoint, I think the single sighting plane of the o/u is probably superior from a firing perspective, but a graceful little side by side cradled in the crook of my arm in the fields just seems oh-so-right.