Moving guys around to jump start an offense is so common it barely merits a comment.
A number of reasons
1) get guys out of their comfort zone
2) screw up the opposing coach's matchup strategy
3) hope for "magic," sometimes a new combo works for a short stretch (see Weal, etc), and the playoffs are a series of short stretches
The first line with Raffl instead of Giroux played as well or better, different chemistry and style, but good results.
Unfortunately, Giroux is past the point in his career where he's gonna jump start another line.
Due to injuries, AV simply can't field 3 solid lines or a top checking line.
And Frost isn't the answer, for him to not even be considered an option, he must look really bad on film, and we know it's not age/experience, or Farabee and Bunnaman would never have seen the ice.
As far as AV "praising" Thompson, HCs rarely throw scrubs under the bus (and the ones that do, often lose their clubhouse), highly paid stars that don't produce are a more complicated matter. If a "star" isn't producing and isn't hustling, failure by the HC to call him out can lead to backbiting and resentment in the clubhouse.