Thanks for your reply. I think it will need to go down as one of the mysteries of TfL. There are of course direct trains from Bank to Liverpool Street too, and the walking distance is only a tad longer (arguably simpler) and well within the distance of the farthest OSIs. DLR is quite a specific case as the exit is equally easy (awkward) to both gatelines. Having thought about it some more, the lack of a Monument/Liverpool Street OSI could well just boil down to the active OSI being for the benefit of Waterloo&City line passengers. The official rationale will have to remain a mystery.