Handing out Barfbags at a themepark attraction is a little bit like a gimmick in a William Castle movie production or a nurse at a coaster station. It "should" be considered real showmanship, but I guess that Disney is indeed serious about it.
I have not yet ridden MS and somehow don´t feel an urge to hop on a plane and endure US custom procedures to ride it (which is exactly what I did to ride Siderman). There are so many mixed reviews floating around (especially from european riders), who don´t seem to be too impressed by the experience.
Not being a "Disney-head", I am the first to admit that Disney rides never fail to amaze me, so I guess I would love MS too. But I guess that handing out barfbags in a family park will probably increase peoples perception of actually feeling sick, as if it is intended and part of the experience.