What my point is, is that yes he mostly guarded the Center ... because he played Center.
Not because Jay didn't trust him guarding the PF, but because it just makes sense to have the quicker, more mobile Power Forward (Ed and Amir) guard the opposing quicker, more mobile Power Forward.
I think what this should be about is, can you find another instance where another Teams Starting Center is assigned the task of Guarding the Opposing teams Power Forward every night? It just doesn't make sense unless you have a Dwight Howard on your team, and can say 'guard the best guy out there.'
You may be right about him guarding the Weaker Offensive Player Tim; but I think its more just how cookie crumbles and not by design.