Well, first of all, software updates also correct bugs. While one may not yet have encountered such bugs, it could happen one day with an older release.
Secondly, Microsoft has done a real good job in terms of making the versions of Office on Windows and Macs as compatible as possible, and updates continue to do that even more.
Third, as you mentioned, there are the security issues that are addressed. While you might be working "isolated" enough now, again there could be instances in the future where such protection is vital.
Fourth, there could be improvements in performance, and that is always welcomed.
I forgot to mention one pertinent thing, though. Sometimes updates are released for specific Mac OS compatibility. I had been using Office 2011 for quite some time before moving to Office 2016 back in October, but I believe there was an earlier release of Office 2011 that was specifically for El Capitan compatibility. Here is an example:
http://www.macrumors.com/2015/10/07/microsoft-office-2011-outlook-fix/
So, it looks like V14.5.5 of Office 2011 was specifically released for El Capitan compatibility. Fortunately, I am not having any issues at all using Office 2016, and I use (and depend upon) Outlook for my EMail needs. (By the way, when I was answering your detailed Excel questions, I was actual getting that information from Excel 2016).
In actuality, if you are using any third party software, you need to insure that it is compatible with whatever Mac OS you are using. That might require downloading and installing updates for such software.
By the way, one of the primary reasons why I upgraded to Office 2016 is that it's just a matter of time before Microsoft stops both updating and supporting Office 2011. Just something for your consideration.