I also think that, when I think of the beneficiaries of the giving as real people and not abstract "lives improved/saved," then "donate now" seems more compelling.
Certainly I could not tell a potential beneficiary to their face that I'm saving the money for the unlikely possibility that I need it myself, but it's fine because if it is too late for them by the time I donate I will probably be able to help someone else just as much or more at that time.
Along that same line of thinking: if my siblings desperately needed financial help and I could afford to give it to them, I would surely not keep the money out of a desire to have more later that I could give to my nieces and nephews.
Certainly I could not tell a potential beneficiary to their face that I'm saving the money for the unlikely possibility that I need it myself, but it's fine because if it is too late for them by the time I donate I will probably be able to help someone else just as much or more at that time.
Along that same line of thinking: if my siblings desperately needed financial help and I could afford to give it to them, I would surely not keep the money out of a desire to have more later that I could give to my nieces and nephews.