I would agree with you in the sense that the minister is the primary person responsible for what goes on in his or her office. I have no quarrel with that whatsoever, but that doesn't mean the committee isn't justified in seeking to hear from other persons in the office on particular matters of which they may have direct knowledge. I don't mean to suggest this is the case here, but you might appreciate that if the government of the day can pick and choose what documents it will provide, or can pick and choose what public servants or what political staff show up here before committees, that has the potential to undermine the ability of a committee or the House to hold the government to account.
So it is that as in a court proceeding, so also here: the person of whom a demand is made to produce a person or a paper does not have the choice of selecting which it shall produce. I fully agree with you that ultimately, and maybe not so much ultimately but at first instance, the primary person responsible for the conduct of the ministerial office is the minister himself or herself.