Deans didn't have this option. Deans had a bunch of in-experienced duds foisted upon him, and he didn't have say in the matter. He didn't get to hand-pick his assistants, unlike Rennie.
How much of this is Deans' fault versus something he had to accept?
It has often been said that a major reason Deans missed out on the All Blacks head coach role to Graham Henry is that he didn't arrive with a concerted plan as to who he wanted as his assistants and his overall plan for the role compared to Henry.
Did the same thing happen in Australia?
Rennie clearly approached the Wallabies job with a set of assistants he wanted to hire and aside from a couple of options as his forwards coach not wanting the role, he did manage to assemble his desired team.
Deans came into the Wallabies job as a heralded provincial coach with a vast amount of success. He was absolutely in a position to make demands about who he wanted around him. If those demands were all refused then perhaps he shouldn't have taken the role?