Alisa - elastic in a casing like this, rather than sewn down, doesn't catch poo like sewn elastic does, for a start!
If you look at a piece of fabric with elastic sewn on by zigzag, the furrows of the stitching are really deep, and don't wash too well (I've found). If everything moves freely it should wash better.
And it might be more comfortable?
Thank you so much, OP - I was jus trying to visualise how to do this yesterday, both the casing and the roll. Now I can make a bamboo HH knock off with a rise to suit my baby!