User blog comment:EvanDude345rv5/Fairy Tail Chapter 358 Review : Magical Barrier Particles/@comment-121.54.22.98-20131103083600/@comment-7418318-20131104002011

Tartarus might have other reasons to go after the ex-councilers than their magic power. Yajima didn't seem like much of a threat to them either, but they still came for him.

That leads me to believe that they are either trying to send a message, eliminate the most likely "replacements" for the dead council or they are trying to keep knowledge that the council had access to from getting out.

Besides, do we really know for sure that Ultear lost her magic? Neither her letter nor her thoughts mentioned that. Considering how many other old people are still very powerful wizards it's possible she still most of her magic.... IF she's still alive.

She did say that she didn't have much time left, so she might already be dead by now.