The drug is toxic, and a max dose gives efficacy but side effects, especially toxicity to organs is a concern. So, fine tuning the dosing and administration (infused how quickly) can alleviate toxicity.
I believe the delay was due to dose/infusion rates and ped dosing. I don't think they will wait for confirmatory phase III. Unfortunately, these patients die, not from Marqibo, but from a very aggressive form of cancer. Marqibo, to those patients, gives them the ability to extend the inevitable. Obviously you want the patient to be comfortable after dosing and have improved QOL or outcome.
I hope that helps.