You could too ask me "what a possession mage can do, besides magic ?", it'll be pretty the same type of question.
The point is possession traditions allows a mage to gain an extraordinary resistance, extremely high attributes without any implant or adept power, a free skillwire for 3/4th of the active skills, and he already have heavy firepower since he's a mage. He probably beats the streetsam mano-a-mano, he's an incredible infiltrer without even trying, since he has a very high agility attribute, he can emulate skills he never put karma in at the spirit's force rate, and can add concealment, he can move with incredible speed, would he have matrix abilities he would be a one-man team.
This make such characters hard to handle for GM, because they break a basic rule which is that all PC must have their place, their role in the team, and when a character's ability are totally overwhelmed by another, it's hard for him to find his place, to have a role in the team.
That's why there is a BP system, to allow characters to have very similar global power, but applied to different areas. It's not designed to allow some character to have one or two areas of expertise, and others to be masters in nearly all areas.
Of course, the GM can handle it, but it's much more hard than when you got a pretty similar global power among characters and no redundant functions in the team.