Black cabs are very necessary.
Aside from the point that the above poster made, that tour companies charging 100 per person means they have groups of 10 or 20 people, and very rarely seem to stick to any advertised itinerary, those of us who live in the suburbs often need such 'illegal' taxis because otherwise, without 24 hour public transportation as seen in other major world cities, it would be impossible to get to and from home.
When the authorities clean up and make better provision of their 'legal' services, 'black services' will no longer be requireds. But until then, I'm glad to have them around.