"Methodological pluralism is the thesis that the use of not only multiple theoretical models but also multiple methodological approaches in the course of scientific practice is legitimate: "Given any rule, however 'fundamental' or 'necessary' for science, there are always circumstances when it is advisable not only to ignore the rule, but to adopt its opposite." -- PaulFeyerabend
"According to pluralists, no approach can be ruled out for good. If, for example, a research program is mainly interested in discovery of new phenomena, a counter-inductive approach (i.e. counter-inductive with respect to current 'facts') may prove successful. However, this does not make the choice of method arbitrary. It does make the choice highly context-sensitive and a good deal more complex than some rationalists imagine. " - from C2.com
Since all methods rely on best cases to pursue and worst cases to avoid, and some assessment of the status quo (what is true right now), it is at least necessary to list and debate those to be able to permit multiple methods to co-operate. It may also be necessary to give threats and visions status, as is done in some agile methods.