If you use the command line to do the update there is no need to reboot, however, the issues that the offline update is trying to solve still stands.

Some programs can crash when all their files are suddenly swapped for new ones. I've seen multiple linux systems bork themselves during an update because the update itself caused the desktop session to crash, which caused the update process to be interrupted just as it was modifying some crucial files, thus rendering the system unbootable.