Ambiguity costs clarity; exactitude costs flexibility.
The more ambiguous you are, the more likely it is that people will have trouble using your taxonomy to find and classify things.
For every ambiguous rule of classification you use or label you hide behind, you'll have to communicate your intent that much more clearly.
For example, what if I had organized the lexicon in the back of this book by chapter, instead of alphabetically? This might be an interesting way of arranging things, but it would need to be explained, so you could find a term.
The more exact your taxonomy becomes, the less flexible it is. This isn't always bad, but it can be. If you introduce something that doesn't fit into a category things can get confusing.
Because there are many words for the same thing, exact classifications can slow us down. For example, I recently tried to buy some zucchini at a grocery store. But it wasn't until the clerk in training found the code for "Squash, Green" that she could ring me up.