This is a follow-up ticket to MGNLSLOCK-32. It actually would be possible to re-introduce polling with a simple one-liner in the start() method, something like UI.getCurrent().setPollInterval(5000). The good thing about the new event-based implementation is that it puts much less load on the server and in our case - with polling enabled - would trigger a UI repaint only in case of an actual update, thus performance issues should be greatly mitigated. However, as Espen explained in the related ticket, being polling a UI session-wide mechanism, there might be cases where another component uses it (e.g. progress bar in file upload) thus causing polling for page changes to stop working (which might be solved with a central registry for polling).
At any rate, it all boils down to
My view: re-enable polling, cause without it SL usability and UX would be bad
Espen's view: polling is disabled cause it may be problematic in some use cases, besides putting more load on the server