How important is the separation of powers? I am talking here specifically about the separation between executive and legislative powers. Judicial independence is a separate issue.
I should differentiate between strong separation of powers and weak separation of powers. In systems with strong separation of powers, the election of the executive branch and the legislative branch are independent of each other (though they could happen at the same time). The American system is a system with strong separation of powers; the president and the legislature are independently elected. Systems with weak separation of powers do not independently elect the executive branch and legislative branch.
In systems with weak separation of powers, the election of one branch is tied up with the election of the other, so the subordinate branch represents the interests of the directly elected branch to some extent. The most common form of weak separation of powers is legislative dominance. Parliamentary systems are dominated by the legislature. When the legislature dominates, it is responsible for electing the executive branch, but there are usually institutional constraints which prevent the legislature from easily replacing the executive branch, so the executive branch has some degree of autonomy.
I have been a pretty strong supporter of the separation of powers based on the theoretical arguments for it. The argument that separated powers will tend to check each other’s excesses because they do not represent each other’s institutional interests is a strong one, but I have never really seen any empirical evidence or strong anecdotal evidence that power separation is a good thing. Do developed nations with weak separation of powers have problems that are attributable to that weak separation?