In their textbook Approaches and Methods in Language Teaching (2nd ed.), Richards and Rodgers (2001) argue the validity of the Grammar Translation Method, stating that
Though it may be true to say that the Grammar-Translation Method is still widely practiced, it has no advocates. It is a method for which there is no theory. There is no literature that offers a rationale or justification for it or that attempts to relate it to issues in linguistics, psychology, or educational theory.
Within a historical context, this statement would be accurate. The origin of the Grammar Translation Method is based on the need that existed back in the 18th and 19th centuries of finding a formula that could help monks, students, scholars, and scribes to accurately translate large passages of text between two complete different language systems, particularly Greek and Latin texts.
Although there is a trace of philosophical foundation in the form of assigning mental tasks to the brain (anything psychological was unheard before the 1800s), the GTM still does not reunite the among of philosophical background that would turn it, from a method, into a thesis. This entails one thing that is mentioned by Richards and Rodgers: it has no theory.
Again, if it had a theory, the GTM would have the philosophical basis that would prompt the analytical inquiry that would prove its effectiveness, and its validity of its application as a method. This is why Richards and Rodgers also say that GTM "no literature that offers a rationale or justification for it or that attempts to relate it to issues in linguistics"
This is also true. When an intervention is established as a practice, the way to test its effectiveness is through qualitative or quantitative research. Instead, GTM has been used since the late XVIII century and taken for granted as sort of a sine qua non method.
Now that it is clearly established that Richards and Rodgers accurately state the lack of theoretical validity behind GTM, the implications for the modern scholar are that a) GTM should not be used in isolation as the only intervention for L2 teaching; b) GTM is a method, and not a "best practice"; c) GTM can help in some processes of L2 learning, but not in L2 acquisition; learning is a conscious and exact process while acquisition of information is a complex process of interaction.