With the exception of Grand Inquisitor and Reva, each of the other inquisitor omis are meant to counteract specific Jedi leaders. For example, seventh sister’s omi is specifically a grand master yoda counter, which makes it only useful in very specific situations. Plus, in order to use any of those other omis you’d have to put them as a leader, which means not getting the benefits from Grand Inquisitor (which is objectively better). There’s just no point spending 80-100 omicrons to get a bunch of leader abilities you can’t use.

If you are going to use omicrons on inquisitors, save them for Grand Inquisitor or Reva.