Because the excess fuel that isn't used by the engine will be dumped back to one wing.
Lets assume the following scenario:
1) L AND R Boost pumps are ON
2) #2 Engine is running
3) Fuel from both L AND R main tanks is being fed through the respective collector tanks
4) Excess fuel that isn't used by the #2 Engine is being fed back to the wing of that respective engine, in this case the right main wing via the motive flow pumps (can't remember the name of the one that is involved in this one... but one of those motive flow ones...)
If you would have had the L Boost pump off, that fuel line would not have pumped fuel to the right engine.
People claim that the auto cross flow system won't have enough time before takeoff to correct the imbalance therefore you will takeoff with an imbalance.