I have a question with the mold issue, actually. Why would it grow mold? Is this usually a problem with mattresses but not mattress atop box springs because the box springs allow air flow? A particular part of the country or weather or part of a building? I can see like a basement apartment or something maybe, but I've never heard of mattresses growing mold before, regardless of bed frame or lack thereof. I feel like I knew lots of people using box spring on the floor type beds in and just after college.
Oh, and I've always done the mattress cover type things on kids beds or cosleeping beds, but once they were in their own beds, I stopped doing it on mine. If I have a sick kid in my bed, I'll put down multiple giant towels and just strip off one as needed. I'm gonna wash everything thoroughly as soon as they're well again anyhow.