Not every Character needs to be good in arena. When people generally talk about buffing characters they want them to be awesome in every area. I have Maul 6* and he is great in GW. I use him in the Arena only when I see a good opportunity vs. Jedi teams.
Looking at how good a character is based on who uses him in the top 5 of arena may be miss guided. A character like Maul may not be in a top 5 lineup for many reasons (Not leveled, Geared, Starred up or not having synergistic team members with him).
Personally I like having specialized characters like Maul that are good for specific situations if used properly. I use him as a leader on a Sith team in GW (Maul, Dooku, Sid, Savage, and Vader) and it is a great team against Jedi. I generally don't use it in arena though because I don't trust the AI to work players in proper combination. I do use it in GW all the time and love the results.