when to merge two databases

The question is: If I've got two (or more) Oracle databases, when does it make sense to integrate them into a single database, and when should they be left as two (or more)?

Background: (You can probably skip most of this without worry.)

The company has several medium-large databases running under Oracle. There is some overlap of data in these. There are various means and methods of getting data in, manipulating it, and getting it out again.

This system grew due to the requirements of various departments doing work in somewhat related tasks. Issues such as billing, debt, collateral, foreign exchange, trades of various products and services, and settlements with the market regulator, all have their own database, or schema in a larger database. There was not a single architected design for the entire system, though there were some pretty good architects on individual projects involved.

Some of these databases have been around for a long time, and even started life in some other product and got migrated to Oracle. Others are less than a year old.

At present, there is a not-very-robust nor well written patchwork of data transmission between these databases. The various powers that be are very unhappy about this and want it cleaned up.

There are two competing pressures here. One "camp" wants to do a lot of work on the interfaces, making them better defined, more robust, and so on. Apparently we've settled on a product for doing this.

Another camp wants to reduce the number of databases by merging some of them. It would, they claim, reduce the inter-database communication issues, as well as decreasing the reproduction of data. (In some cases, entire tables are lifted from one database and copied into another.) They want to do integration work to make the databases serve the larger needs.