Both of those reasons seem to factor in with this song. I realize they've got to give Mint something to do to keep her place in the group. But forced, terrible rapping can't be the only option.
If a member can't sing, have him or her dance. If he or she can neither dance or sing or just dancing ain't gonna cut it, honestly, that's fine — just as long as you don't label them a rapper solely because of that. Teach the member a skill. I'd much rather watch a video where one of the members was just painting a portrait in the background than listen to another painfully forced kpop rap.
There are plenty of options: miming, shooting baskets, target practice (an option I'd love Sistar's company to explore. Just have Bora shoot arrows in the background the entire MV. No raps, just arrows.). The possibilities are limitless.