For our agency, which also has a small GIS infrastructure group, which are also part of our <10 full-time GIS), but with about 300 part-time ArcGIS users, and of course many GIS consumers, I haven't found a need to fire up an internal Portal yet. There may com a time where I would like this. I see Portal as being a need if you need to keep things 100% internal or within the firewall. But there are other options, like just using the security within ArcGIS Server, to accomplish the internal-use only that we need.
The other place I can see that it would be handy is using some of the web apps (Web App Builder) and many of the other apps and software that are becoming more reliant on AGOL/Portal to authenticate their use, whether just because it needs a (public) basemap, or it needs a named user). For better or worse, this does seem to be the way all things are heading. We do not have enough named users to allow all to access via a log in, so then we have the choice of the AGOL-public option or again, looking at internal security options. So far, we've been using the internally controllable options (thru ArcGIS Server), but that could change as we try to make more of our data/services in an available format for the public (a work in progress).
I guess another option would be is a split between the too (names users, some in Portal, some in AGOL), if for no other reason but to take advantage creating an http://opendata.arcgis.com/ site. Again, even for OpenData, I see keeping the control, and maybe the data itself local (thru .zip shapes and or GP services), while providing a public metadata and link.
Anyway, just some of my thoughts on the subject. Always good for me to see what others have done or are planning too.
