Hello SSP,
you are basically right and wrong at the same time. At the moment and to my knowledge, there is no way of measuring random 3D-Area (without a workaround, see below), BUT Agisoft is able to calculate the Area projected onto the best fitting plane of a polygon, which sometimes is even more usefull.
This means you can measure the roof area in your example by making two polygons, one for the west-facing roof side and a second for the east-facing roof side (and then add the areas of the two). The result is calculated in the field named "Area, fitted 2D". The "problem" in your example is, that the best fitting plane calculated by Agisoft in that case correlates with the XY-Plane an thus delivers the same result for 2D and fitted 2D.
When measuring a wall, like in the example that I'm attaching, you can see that the 2D-Area is basically 0 (vertical wall!), while the fitted 2D is what we would call 3D area (but projected as a 2D area to the best fitting plane) and that is the real wall-area.
Workaround: If you select only that part of the Mesh that you want to measure, then Edit --> Invert selection, then delete selection, you basically cut out all other parts of the Mesh and only leave the Roof untouched. Then you can calculate the total amount of area by using Tools --> Mesh --> Measure Area and Volume.
Feature Request: Nevertheless, you make a good point, that it would be very useful to be able to select Mesh-Faces and then be able to calculate its total 3D-Area.
Hope this helps and we get this as a new feature soon, but I do not want to miss the 2d fitted area feature!