That was my first thought, too. My ideas was to make the plastic UV-curable so it would become rigid and not require inflation. I looked it up, and that's what they're doing - UV-curable ribs. It's also not a sphere; it has an inflatable ring. Pictures & paper here: https://asteroid.arizona.edu/KABAND_Inflatable_v3_public.pdf

I guess it depends on the odds (impacts per cross-section per time) for different debris size/composition/speeds in the planned orbit. I'm sure somebody's studying that, but I'm also sure it must involve a lot of complicated tables.

The particular mini-satellite is slated for a 6-month mission, so presumably they think it'll survive at least that long.