Brent Ozar Unlimited had a post recently noting that it appears Microsoft has seemed to move away from releasing service packs for SQL Server and instead is only releasing cumulative updates. Given that Microsoft itself warns against installing cumulative updates unless it solves a specific problem you are encountering, most DBAs, including myself, are somewhat reluctant to install them, preferring to wait for the (supposedly) more thoroughly tested service packs.
A recent Steve Jones editorial has picked up the call for service packs and includes links to Microsoft Connect items to vote for releasing service packs for SQL 2012, 2008 R2, and 2008. Please take a moment and vote for these.
Release SQL Server 2012 Service Pack 2
Release SQL Server 2008 R2 Service Pack 3
Release Final SQL Server 2008 and 2008 R2 Service Packs