Each version of SQL Server had had at least one SP or CU that within a week or so gets pulled or updated for some reason so I am hesitant to apply them right away. If we were to keep up-to-date with these we would have to hire more testers and another DBA with the sheer number of database servers we have. Also, for purchased apps they don't test against the latest SP or CU for older versions of their software and a lot of vendors will not support you if you have an issue if you apply these. We try to keep somewhat up to date. We DO apply the security patches supplied for SQL Server within a month though.