Field no accessable to specific groups, default value not working

[ZiNgA BuRgA]

From memory, the default value only affects the thread create page (i.e. what is shown to the creator as default).  It doesn't actually affect the underlying value.
If the user cannot edit the field, the default won't have any effect as they are not allowed to set a value at all (even the default).  Yeah, it's a bit confusing, but using the Blank Replacement value is actually the intended way to handle it.