I'm going to keep my POV short.
If the company views the idol as useless, then the audience will view them the same way.
Most of the time, when a certain member is called a "filler" it's due to the lack of exposure that their skills have thanks to the company's innability to give them a bit of spotlight (which doesn't necessarily mean that the idol is being "mistreated", they're just being mis-used).
There are a few groups that do have filler members, but overall, it's a word that's thrown around too much.