Actually, I'd prefer a better system than automatic synchronization. Sometimes you don't want same extensions on all systems. Instead, it should sync all extensions and present them to user as "User Addons Library" that is the same on all systems. And user just has to click on which extension he wants to use on a particular system. This way the selection of addons is presented to you as a collection of all addons you're using on all your systems. But you "install" or approve only those you want on a particular system.
This way you have the convenience of easily selecting between addons you like and ones you actually want on a particular system without having to manually through addons library and manually installing addons you want. Because full head on addons/extensions syncing is annoying and tends to get broken. As seen with Firefox that does that and totally freaks out over time as you're fiddling with addons on different systems. Or Chrome which syncs them alright, but the same on all systems whether you like it or not.