I was under the impression that replication was planned so that users on the osm-ts would have access to the DB without hammering the main DB (which would be there for production tools) with expensive queries.
Or so I gathered from our talk on IRC on the toolserver channel, but may have misunderstood you.
There will definitly be no direct access to the main db. from what i gathered at the dev meeting, the idea was to let people import the regions they were interested in on the map TS. No replication, no full data, nothing.
But if replication isn't an issue space-wise, why not? It's sure cooler to have the full planet data to play with, in real time.
-- daniel