from the key to North’s Prose Composition 7.9:
ὁ σῖτος [or, τὰ σῖτα] καὶ ὁ οἶνος εἰσι χρήσιμα τοῖς ἀνθρώποις.
Why not χρήσιμοι? Is the thought that they are useful things? The English: “Corn and wine are useful to men.”
Shouldn’t it then be ἐστι χρήσιμα?
The subjects would be masculine even though the prædicate were neutral.
Indeed. I initially confused between the subject and predicate.
In any case, you’ve answered your question. No reason to use the masculine here
for the predicate noun. While the subject grammatical gender is masculine, they’re neuter
in meaning and would be better described with a neuter predicate.