The inkbrush has been used in competitive play, it's just not that common to see.
However it depends on the brush, the octobrush, the octobrush nouveau, and the permanant inkbrush have all been pretty successful.
There is a competitive player who goes by the name of Silver, and he's currently on the team Komodo. Silver specializes in brushes, and he is very good with them. However if you're looking for a vanilla inkbrush or the inkbrush nouveau I don't know what to tell you. Hope this helped answer your question.