@pelson explained that the purpose of omitting to plot the last gridline of a circular coord is in case it overlaps (on certain Axes) : An "overdraw" can make it look odd (with any slight projection error, or alpha transparency).

But if we omit the last point from the *labelling*, this will definitely look all wrong on a standard rectangular plot
   - e.g. `-180, -90, 0, +90, > *nothing* < `

The code as it is (i.e. not my doing) makes a simple guess as to whether such an overlap *might* occur, based on the coordinate circularity -- it doesn't actually work out whether it **does** :  That would depend on the relationship between the crs of the gridlines and the Axes projection, and I don't think we actually know how to do that.
Obviously this whole area is a bit awkward at present ...  
@rhattersley, do you know more about these problems and the current state of play?