In the United States, Cabinet members must be approved by the Senate. This is relatively uncommon, since in most countries filling out the Cabinet is considered the President's prerogative. After all, secretaries and ministers serve at the President's pleasure; they advise him and help carry out his political agenda. For example, though Argentina's constitution is based on the U.S. constitution, the President does not require approval from the Senate when naming his ministers.
In the U.S., however, the Senate can outright reject the President's nominees for Secretary of Defense, Secretary of State, etc. The goal is having a Cabinet which commands broad political support, not one composed of marginal or excentrical figures. It stands to reason that if political advisors become more important than Cabinet members, the Senate's influence will be reduced, and the checks and balances weakened, since the President can designate whomever he wishes as a political advisor, no matter how extreme.