Each node selector, subject hierarchy view, etc. widget contain a scene model, which may take significant amount of time to rebuild from scratch after batch processing. If you load the brain atlas scene (it contains 1200 nodes) then each node selector adds about 5 ms, each subject hierarchy tree view adds about 15 ms to the time to get out of batch processing, which can easily add up to hundreds of ms. It may still be hardly noticeable, but if modules started to use batch processing to try to speed up simple operations then it would actually slow down the application.
I fully agree, though, that there are much more important things to improve in landmark registration.