A cyclic process is important. It gives more chances to learn from experience provided that there is real reflection on the process and on the outcomes, intended and unintended. Qualitative information is less constraining of the process. Participation is a somewhat different issue, more to do with action than research. Action outcomes can usually be achieved only with some commitment from those most affected.
Participation is one means to that commitment.
There may well be other reasons, too. On some occasions the eventual interpretation of information is richer if involvement is greater. So far, I have taken the view that action research can take many forms. There are some conditions, however, that I believe are more important. I think it is important that the evidence is used critically rather than uncritically.