Safari updates have always required a restart because they install an updated version of WebKit (the framework) into the system, which is separate, but used by Safari (the browser).
I run WebKit.app (the nightly development build of Safari) and they've started doing delta updates right within the browser itself. Rather than download the entire thing every day, they send just the bits that have changed. Updating has been so much quicker. I hope they bring delta updates to more parts of the software update process.