One advantage I can think of is access to a pretty good speech engine - with Linux, you have to pay for something of reasonable quality (eg Cepstral voices).
The value is being able to run programs that can only run in Mac OS X. There are plenty of reasons why you might want to do this. You might want to build some custom processing job that relies on something built on top of OS X for example. There are at least two YC companies that have done this.
Do you mean the command line differences? Or will people who use this effectively screen-share the Leopard server and see it "as is." I know very few developers who demand that.