A correspondent interested in linguistics terminology asks: 'What is the difference between the terms in such pairs as acute and acute accent, compound and compound word, benefactive and benefactive case, past and past tense, and so on? Are these variants near-synonyms or synonyms? When should one use two-word terms and one-word terms? Is there any regularity?'
The abbreviation of a compound word or name to its first element is very common in English, to achieve economy of expression, but it's only likely to happen if there is no danger of ambiguity. It's therefore especially common in technical expressions, where the meanings are typically very specific. Benefactive has only one meaning, so in most contexts there is nothing lost when case is omitted. But a related factor is whether the second element contrasts at all. As the notion of benefactive developed, it began to appear in a wider range of contexts, with such expressions as benefactive role and benefactive implicature being used. The more options there are, the more it could be important to use the full form of the expression to avoid ambiguity.
Whether someone chooses the expanded or reduced form is partly also a stylistic matter. I often use an expanded form on first use and then switch to a reduced form, to avoid repetition, but other factors can intervene, such as rhythmical elegance and awareness of a possible lack of clarity. For example, 'The past tense of walk is walked' is slightly clearer than 'The past of walk is walked. Why? Because past has so many meanings, there is in the latter example a fraction of uncertainty as to what is meant. It quickly resolves in this example, of course, because the context immediately makes it clear. But the risk of ambiguity is always there: 'The past often presents a problem to learners'. In these cases, we have to work harder to determine what is meant.
Abbreviation makes the writer/speaker's job easier, but it makes the reader/listener's job more difficult.