in survival, you can left click with anything on a note block to hear what note it is without changing the pitch. you can right click the note block to change the pitch.
starting somewhere around 1.6, in creative mode, holding a sword prevents you from breaking blocks. however, it doesn't allow you to hear the current pitch of a note block. because of that, there is no way of directly hearing the current pitch of a note block in creative mode, where it would arguably be most important.
it would make perfect sense that hitting a note block with a sword plays the note block. this doesn't interfere with anything and would be simple to implement. it would make composing / creating songs with note blocks a lot easier.