The attitude 'I don't have to speak to the MATZ if I'm near or passing through, so I won't' is unhelpful and potentially dangerous.
Did anyone say that? The context of this question was that you're receiving a traffic service from someone else and wearing their squawk, thus immediately easily identifiable to and contactable by the MATZ controller should they happen to be interested.
If you were listening out to London Information with the volume turned down and the transponder turned off that would be a different question to which different answers might be appropriate.