Currently, it's possible to get the title field from the question type returned by /questions/{ids} and from the answer type returned by /answers/{ids}, using the appropriate filters.
Unfortunately, the post type doesn't expose this field, so if you're interested in the post title for a list of question and answer IDs (useful for display purposes), you're stuck making two separate requests instead of using the otherwise convenient /posts/{ids} route.
Could the post type be updated to return the title field when supplied an appropriate filter?
