Mixed methods, generally, should only be used when there is a real need to employ both qualitative and quantitative methods to answer your research question(s). There are multiple types of mixed method designs, and the design chosen should be justified by the context of your research.
http://www.stiba-malang.com/uploadbank/pustaka/RM/RESEARCH%20DESIGN%20QUA%20QUAN.pdf CHAPTER ELEVEN Mixed Methods
Q1) Described when mixed methods research is appropriate.
Q2) Justify research designs when appropriate.
Q3) Examine the role of mixed methods research in formal inquiry.
Q4) Assess the relative strengths and weaknesses of each research methodology