Agreed.
I think the reason Garrison hasn't been played on the right side is because that is where the coaches expect him to settle. What they want him to get is experience playing left side with other players in case of future need.
I think the basic plan is to have 3 of each of the top 4 be able to play either side. LH-->Edler, Garrison, Hamhius, RH-->Bieksa, Edler, Garrison. Is obvious to me that the coaches have no faith in Ballard on the right side and Tanev, though steady enough to play anywhere, is a natural righty.
Since Garrison is probably expected to settle on RH D they may as well get his practice over early.