I agree with you in the sense that I would never use my definitions without any qualification if they differ from the commonsensical definitions. That would defy the most basic rules of communication. I will always take the effort of clarifying the difference. This effort is worthwhile because I have found that, often, a large part of the confusion in a discussion results from wrongly defined terms. So just offering a different set of definitions can help to clarify the discourse and can thus result in new insights / less conflict. We don’t have to be a “passive receiver of linguistic history” because conflicts are often aggravated by word usage based on misunderstandings. We have to engage with the language and redefine it wherever necessary / useful.
For example, I have found it useful in discussions with non-believers to explain to them my conception of the difference between “faith” and “religion”. Another example is how John Walton criticized the currently used conceptual separation between “natural” and “supernatural”. There are countless examples where progress in understanding can be made by correcting the terms that are currently causing confusion.
In a similar fashion, using the term “creationist” for the specific category of YECs equips them with a title that perpetuates the misconception that they are the only group that believes in Creation. In this sense, I completely agree with @Professormom that appropriation of the term “creationist” by YECs has a marketing purpose for YEC ministries. In my opinion, this is a completely valid reason to share a different set of definitions. Of course, this different set has to be introduced appropriately.
Besides, I think that the definition of “-ism” and “-ist” that I have given fits very well with the connotation that underlies the usage of these suffixes as I have seen in the discourse on origins and in general.