Currently, the portal's Documents & Media service processes all the logic used by Liferay Sync. We can instead use the MessageBus and offload the processing to a new Sync plugin.
The main advantage is having an independent release cycle. New features requiring changes on the server side (new web services, new administration portlet page, etc) will always need to wait for the next release of the portal. By creating a separate plugin, we can make updates to the code and push to marketplace as needed instead of waiting for new portal releases.
We also gain speed and robustness. The processing is offloaded to another webapp thus it won't bog down or crash the portal webapp.
The downside is that customers wanting to use Liferay Sync will need to install the plugin (similar to Social Office).