Plausible Deniability is certainly a reason not to use regular military forces. It's one of many. Ultimately it's all part of the same overarching framework. The question: why hire adventurers? includes the prefacing question: why aren't regularly military forces being used for this task? There are numerous possible answers to that question, some of which are more plausible than others depending on the specific circumstances of various cases. It is important, for world-building reasons, both that there be an answer, and that the implications of said answer be considered.