If for no other reason than it kills off some end-user time, hassle, and bandwidth, it's useful.
Also, a lot of the people I've talked to are touting the crossposting bit as one of the killer features of DW. Considering the inherent laziness of people (myself oh so definitely included), I can think of a lot of people who would love it if they can do it from an endpoint client as well as the web-based client...