The Knowledge is outdated and is easily replaced by GPS. Of course you need local knowledge but not 30,000 streets.
I rarely use black cabs as London's public transport system is excellent. The only cabs that I have been ripped off in were black cabs. Taking the long route or having no idea where they was going (Heathrow to Twickenham would you believe). The legally required to take you stuff is bullshit and you know it!
The design of a black cab is perfect for the job. Seats 5-6 people with luggage and easy to get in and out of.