Words can become less useful when they attach to too much as well as too little. A perfectly drawn map that indicates only the position and exact shape of North America will often be less useful than a less-accurate map that gives the approximate location of its major roads and cities. Similarly, a very clearly drawn map that does not correspond to the territory it describes is useless. So defining terms clearly is only one part of the battle in crafting good arguments; you also need terms that map well onto the actual territory and that do so at a useful level of generality.

The problem with the term "rationality" isn't that no one knows what it means; there seems to be wide agreement on a number of tokens of rational behavior and a number of tokens if irrational behavior. Rather, the problem is that the term is so unspecific and so emotionally loaded that it obstructs rather than furthers discussion.