The purpose of this issue is to modify how exceptions are displayed to the end user (in the UI I'm assuming). The patch modifies the behavior of ALL FreeMarker exceptions, not just the ones involving the UI.
FreeMarker is also used to print reports and import data (via WebTools).
So, let's say you've configured your installation to display little or no information when a FreeMarker exception occurs. Then you try to print a report or import some data and a FreeMarker exception occurs. With the current patch, you have no way of knowing what went wrong (since you turned off the display of exceptions). You would have to change the properties settings, clear the cache, and try again.
That's why I suggested putting the exception handling properties in the context. The screen rendering context would have one set of properties, and report generation or XML import services contexts would have a different set of properties.
Maybe I'm being too picky. It just seems to me that the current patch might cause confusion down the road.