Oldstyle figures (at least theoretically) are more legible and groups of them make more distinct "wordshapes." And the reason to make figures tabular (i.e. monowidth)--to insure they line up line-to-line, vertically--isn't urgent in the horizontal dimension.
It is for historical reasons that lining numbers strike us today as the default form and thus oldstyle figs are a special case. But one could argue the opposite, that is, that it is lining figures that only need to be brought out in particular cases. From that point of view the question might be what is the point of lining tabular figures?
Sorry, didn't see your question before. As I understand it, the topic of this thread is the use of "opened corners" in constructing shapes in a font editor. It seemed to me like your reply, however reasonable, didn't relate to that at all. Correct me if I'm mistaken.