When querying a number of titles in a singe request, especially when it is expected that some won't exist, it would be very useful to be able to access the results in the same order the titles were in.
Currently the "pages" in the results are sorted by id, with non existant ones having an invented negative id, thus the order of results is usually different to the order of requests.
It should be much less work to add an index field to api.php compared to iterating through the results with lots of string operations at the local side.
"index" could be a new field next to "pageid", "ns", "title" etc. It seems so low cost that it need not even be optional.