In fairness, Himmelberg doesn't want to play defence, similar to Logue not wainting to play forward at Freo last year (and I'm sure countless other examples)
Obviously what's best for the team would override player positional preferences, but I'll disagree with the majority that it's clear and obvious where Himmelberg should play. Playing him in defence hurts Cumming (and Haynes etc) whereas I assume he'll hold out one of Cadman/Riccardi in the forward line until they are ready to go, which is probably best for the team.
All that being said, I'm surprised Himmelberg doesn't want to play defence after his good form there, but he said himself he'd rather play forward.