I think this episode supported my theory that Danny DeVito isn't needed in this show. I don't think he necessarily takes anything away from the show, but I'm not sure he adds anything either. Although I do like the dynamic between him and Charlie.
I like Frank, but I'm glad they've reached a comfort level where they don't feel the need to use him in every episode. There's been a couple this season like that. Like the "Gas Crisis" episode, this one gained momentum leading up to the second review, which tied everything together. And having Charlie grab the hammer on the way out the door at the very end just sealed it.
In any case, DeVito fits in well with the ensemble, it does not seem forced, and when they do not have a use for him, they do not write him in. Sounds about right to me.