I noticed that the "Find in the whole song" radio button is selected and "Replace all" checkbox is checked by default, but Replace By: is not checked by default. It would seem logical to me to have this checked by default as well since it needs to be checked in order for changes to take affect (unless this has been changed and I was not made aware of it, in which case the checkbox can be completely removed).
As the dialog also serves for finding, not only replacing, it would be a bad idea to have this enabled by default (it breaks finding). So at the moment, this is not possible.
That is a good point. I guess that I had forgotten that part.