sql_multi_insert() is used by the functional test framework to add styles to the database. However, only one style is added at a time, so a multi-dimensional array is not passed in.
https://github.com/phpbb/phpbb/blob/develop-ascraeus/tests/test_framework/phpbb_functional_test_case.php#L425

The flip side of this question is, should the else clause of sql_multi_insert() allow for single dimension arrays to be passed in like it does for DBMSes that support multi-inserts?
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/db/driver/driver.php#L514