initially I felt the same way as you do. he was a part time Oriole. but he managed to almost catch the winningest pitcher in O's history. that's a big accomplishment considering he only played half his career in orange and black.
I suppose you have to consider the perspective of why he's being inducted. if it's to honor him for his sake, then forget it. Mussina was never a player who wanted attention. if it's to honor the team, then it makes sense to induct him because he is among the all-time ranking pitchers. they aren't celebrating his whole career, that's for the MLB HOF to decide on. they are celebrating his time in Baltimore. I think most of the rejection he gets is all about his decision to go to the NNY. if he signed with the Indians or Phillies, I doubt many people would be upset with this decision.