Perhaps in these days of wireless beads, they're not. My experience with them is that they 'feel' more supple when handled (some time ago though), right up until the point you need to try and get them on the fecking rim. Rolling resistance, sure, on the road, but cx, not sure it makes much difference and other factors are more important, like being able to ride them when they're flat.Tubs are no more "supple" than clinchers. Further, they have a higher rolling resistance than clinchers unless they are glued on with lacquer (hard glue) and not contact adhesive
I can't comment on the last bit because I have no experience of cx.