The problem is demonstrated in
SI-5889, in which reflectField is passed a method symbol and bad things happen.
Well, you can say, implement a sanity check and don't pass non-field symbols to reflectField. But that gets ugly very quickly, because to get a handle on a field symbol one needs to append nme.LOCAL_SUFFIX_STRING to the field's name. To put it in a nutshell, the "field symbol" is not a good one.
Another idea of fixing the problem would be to do away with FieldMirrors and require all field accesses to go through accessor methods. Unfortunately, this won't work either, since not all fields generate accessors (e.g. private[this] ones).