Yes, sports commentators should be impartial as to the team they are rooting for in a particular game. Commentators are supposed to report the action on the field or court and give background information on players and teams. They should not indicate which team they prefer. That detracts from the viewer's enjoyment of the game.
I think that being impartial as a sports commentator is the number one rule of the role. It is very unprofessional to exhibit bias while commentating on a sports game. Everyone has a favorite, but in the role of commentator it is so vital to show impartiality at all times
When someone turns on a sporting event, they want to choose the team that they root for. They don't want to have a commentator that supposedly knows everything telling them who to root for. It's up to the fans who they see and who they like. Commentators should respect that.
I think that it would be fun to see a sports commentator's favourite team lose a game, or hear them get excited over winning. It shouldn't be for all games but if a commentator likes one of the teams that is playing, or if both commentators like opposing teams, it can create some good topic of conversation.
Sports, in general, are entertainment. Therefore, the norm of impartiality that guides the general media should not be applied to sports commentators. Sports commentators are hired to commentate and entertain and both of those things are best accomplished when the person commentating is not burdened by the restrictions of impartiality.