The most “sane” explanation is that revealing his identity would compromise ongoing operations.
Something like someone who did know him as his secret identity would see his face, hear his association with MI5 and go “Wow Dude! Turns out ‘Niegel from Birmingham’ wasn’t really ‘Niegel’ at all! Didn’t he introduce us to Tommy as his old childhood friend? Maybe we shouldn’t trust Tommy anymore either. In fact why don’t we dangle Tommy from his ankles until he confesses?” So revealing that this miscreant is MI5 could put the life of other agents in danger too.
Or alternatively it can be about protecting some method. Like this terrible person introduced a bunch of criminals to a “secure chat” application, and you don’t want them to think it is not as secure as they think. (Obviously the names and particulars are wild guesses with no basis in reality.)
Not saying it is a great argument, but that is how these kind of agencies think sometimes.
Something like someone who did know him as his secret identity would see his face, hear his association with MI5 and go “Wow Dude! Turns out ‘Niegel from Birmingham’ wasn’t really ‘Niegel’ at all! Didn’t he introduce us to Tommy as his old childhood friend? Maybe we shouldn’t trust Tommy anymore either. In fact why don’t we dangle Tommy from his ankles until he confesses?” So revealing that this miscreant is MI5 could put the life of other agents in danger too.
Or alternatively it can be about protecting some method. Like this terrible person introduced a bunch of criminals to a “secure chat” application, and you don’t want them to think it is not as secure as they think. (Obviously the names and particulars are wild guesses with no basis in reality.)
Not saying it is a great argument, but that is how these kind of agencies think sometimes.