Many of the Defiant
missions dealt with the fate of Bajor, and the presence of a representative was quite justified. Dealings with the Dominion in turn justified the presence of Odo on occasion. But there are always episodes where either one or the other should have been left behind, and sometimes perhaps both.
We could simply argue that since there were so many times it was necessary to have Kira or Odo aboard, a standard arrangement was created to cover for their absence - meaning they ultimately had no reason not
to come aboard!