Was Mumbai a dry run? Dress rehearsal? Al Queda did a rehearsal of their 911 attack in Nepal and Afghanistan to prove the theory that the authorities would negotiate with terrorists after they killed a passenger and allow the plane to keep flying. After that attack they were convinced that they could take over an airliner by killing passengers and crew. They also convinced their own attackers that they had a plan to survive and escape.
There is no doubt an attack like Mumbai could spread terror and confusion in a big city and confuse the responders with multiple mobile attacks.
Conjecture: muliple and simultaneous attacks on soft targets like schools, sporting events, theme parks.