This one of the enduring themes in ScubaBoard threads.You rarely if ever need to ditch all your lead. At depth if the BC fails you try to swim up. If you can't, you drop one pocket and chances are that is more than enough for you to start to get positive. Once you start to get positive you keep getting positive.
We have had threads that ask under what circumstances you would drop your weights, and as the discussion goes along, it becomes pretty clear that there really aren't very many situations in which you would need or want to do that.
Then we have threads in which people point out that in the overwhelming majority of dive fatalities, the deceased diver still had his or her weights, with the implication that if they had only ditched their weights, they would all be alive today.