User blog comment:Miskos3/Fairy Tail Chapter 486: The Fourth Guest, Review/@comment-7418318-20160525151848/@comment-7418318-20160526210959

Kagura had her clothing cut, but was otherwise unharmed, so Ultear would only have saved her some embarrassment.

Wendy would have been killed without Ultear's intervention.

If Ultear's ability to interfere is limited (which it looked a lot to be) and she can only do it once every few days, it'd make sense to save it for either the perfect opportunity to get rid or Dimaria, or an emergency.