The dramatic neccesity of having such a complex character as Rosalind is, first of all, that one to create a continuity and a bond between acts and the other characters in "As You Like it".
The power of Shakspeare's plays does consist in creation of complex and memorable characters (e.g. the Hamlet's speech captivates the audience, because it describes an ambivalent human nature, seesawing between revenge and ambiguity over his own madness).
Rosalind is either the main character of the major conflict, or she is creating the conflict itself. She is a complex human being, having the wisdom to rise above her strong feelings, emotions, to see beyond "idea of love". She is a very interesting interlacing between delicacy and boldness.