What is the advantage of knockout over gene therapy in mice model in order to st - (Sep/16/2013 )

Hi, 

Am going through the literature of TFPI-2 protein, where some people have studied its role in apoE knockout mice by replacing TFPI-2 gene by gene therapy..thereby upregulating its expression, and later also downregulating it via respective vector constructs. 

Of approximately, 10 years of study of this protein no one introduced any knockout mice model of TFPI-2. Recently, morpholino based KO was shown for TFPI-2 though.

My question is: is it that gene therapy/replacement of original gene is a sort of alternative to KO in order to study the role of novel proteins? What is the difference, and does one have advantage over other?

May be the question is kinda silly, but I really want to clear my basic understanding.

Thanks in advance.

Replacement is serving your KO purpose with additional required gene parameter to study.