You probably don't have to go into why you're interested in a particular specialty. I'm sure most people don't know yet, so it's not as though you'll seem unfocussed. On the flip side, if all you can talk about is eventually going into specialty 'xyz' because you've had tons of volunteer/work/jobshadow experience with these specialists, you may come across as somewhat narrow-minded.
I would definitely mention if you're interested in a particular specialty, and definitely mention why, but don't dwell on it. You can even say something like "I am definitely interested in blah blah blah... because blah blah blah... but I'm also very much looking forward to the chance to be exposed to so many other kinds of medicine..." Make it seem as though you can't wait to get in there and learn from everyone you meet, and don't make it sound like you don't have time/respect for people working in other specialties.