Jack,

I understand what you mean. However when I thin of the Locrian mode, i don't really think of it it terms of usage at all.

I don't see it as an option nor as an alternative melodic pattern. I see it as describing an area of the territory cobered by a particular key.

When we think of the key of C Major, we tend to think of a scale. For melodic instruments that deal in music linearly, this makes perfect sense.

But when we start to look at music vertically, each key begins to look more like a field rather than a line, so we see the range of harmony options in all their inversions, including modal ways of viewing the same group of notes.

I perceive it as being one furrow in that field, though I do not ever refer to it, though I know exactly where it is in relation to the rest of the key field.

I think that the main reason it is not used is that, unlike the other modes, it has a flat 5, with the result that it is very difficult to establish its tonality with any conviction.

To clarify - if a tune is written in D dorian, a strong D bass will imply a D tonality, from which the 'colour' notes of the dorian will not disract the ear.

In D Locrian mode however, I suspect (without having tried mind) that the absence of a natural 5 would make the D sound unconvincing as a root and the listener would probably feel unsatisfied if it didn't ultimately resolve up a semitone to the tonic of the parent scale.