Starting May 2014, Windows 8.1 without Update will no longer receive security updatesTechNet blog
Since Microsoft wants to ensure that customers benefit from the best support and servicing experience and to coordinate and simplify servicing across both Windows Server 2012 R2, Windows 8.1 RT and Windows 8.1, this update will be considered a new servicing/support baseline. What this means is those users who have elected to install updates manually will have 30 days to install Windows 8.1 Update on Windows 8.1 devices; after this 30-day window - and beginning with the May Patch Tuesday, Windows 8.1 user's devices without the update installed will no longer receive security updates.
This means that Windows 8.1 users - starting patch Tuesday in May 2014 and beyond - will require this update to be installed. If the Windows 8.1 Update is not installed, those newer updates will be considered “not applicable.”