Is it fair to say that disk-spanning is only practical in "client-server" areas, where the DB server and application server (or more generally, where the database server and the tier where spanning will take place) are on the same physical domain?Not at all. It depends where it makes most sense to keep large persistent indexable datasets, especially relatively static ones. That may be in the middle tier or even the front end. The data doesn't have to come out of a C/S database, either.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
