Separation of Powers: Functionalist View vs. Formalist View

 On page 379, the authors of our casebook say this about the Court's separation of powers jurisprudence:

"Much of the Court’s separation of powers jurisprudence reflects a clash between two contrasting methodological approaches: a “functionalist” view attentive primarily to pragmatic  reasoning  focusing  on  real-world  effects,  and  a  “formalist”  view  attentive primarily to definitional reasoning focusing on categorization."

The "formalist view" appears to be what some would called a textualist view or an originalist view. 

What are your thoughts about how the Court should interpret the borders between the Legislature, the Executive, and the Judiciary? Should the Court police those borders as set forth by the Constitution? Or should it shift those borders based upon the Court's pragmatism and the its understanding of the requirements of the "real world"?