Probably just a feature creep cascade. "Hey, can you make it so we can
access data from TWS via DDE?" "Sure!" "Hey, can you make it so we can
place orders using the DDE interface?" "Sure!" "Hey, DDE sucks. Can you
give us an ActiveX interface for all this?" "Sure!" and on and on.
Yes, plus it allows them to "add features" without touching the back end. I would think it's a smaller and more-contained project to add features to the client rather than fiddle with the servers.
