I think they've been very clever here, intentionally or not. By carrying out the rehearsals, pax will see how busy the stations are and be deterred from using them during the Olympics, therefore reducing the congestion during the important times and reducing the people using the station.
People were advised to use other stations nearby, such as Waterloo East, but I think people would just not listen to the advise and continue to use their normal, busy station. Hopefully, by seeing the disruption today, they will use other stations, reducing congestion.
Then again, knowing the mentality of commuters, they'll probably still use London Bridge.