Barry,
From what I have read, the convenience rollup (KB3125574) is more like ...
a) a collection of all Windows Update patches that have been released for Win 7
If you have been keeping up with the monthly Windows Update patches (minus the patches that you have been skipped because they enable Windows 10 nags or skipped because they enable sending telemetry to Microsoft, etc) ... then you do not need to run the convenience rollup (KB3125574).
If you are installing a fresh version of Windows 7, then the convenience rollup (KB3125574) might make that task easier.
Here are two links to more information ...
Woody Leonard's blog is always a good source of information about Windows issues.
This thread (from Woody's blog) discusses actual experiences with the convenience rollup (be sure to read the comments).
http://www.askwoody.com/2016/windows-7-sp2-convenience-update-rollup-under-examination/
This site discusses know issues with the convenience rollup ...
http://windows-update-checker.com/FAQ/ConvenienceRollupKB3125574-Issues.htm