Well, it says each build expires in 60 days so as long as you keep upgrading your build (which is says happens automatically), you'll be fine. And the last build will last 60 days beyond the new release. Since I have Office 365, I'll get final release the moment the preview is over.
----------
If you've bought Office 2011 instead of Office 365, you're right, you wouldn't need this necessarily. But to me, 365 is totally worth it since you can get it on sale for about $65/year and that includes unlimited storage in OneDrive. To me, it's a steal at that price.
Ah I see.