I personally saw in some cases and heard a lot of similar cases where a patient is struggling for life due to the shortage of same group blood. In many cases the blood banks or hospitals might have other group of blood except the one you needed. For that you will end up searching a lot for the same blood group. I regularly gets emails from my internal emailing system of my company, requesting employees for the required blood group. My company is having around 10000+ employees in bangalore (india), so some body with kind heart would donate the blood if it suits. In many cases it won’t happen because of below reasons.
* The person is not ready to donate.
* The person might have already donated in the last 2 months
* The same blood group is not available.
  According to “newscientist” the last scenario may not occur in future. The scientists in University of Copenhagen in Denmark found a way to convert the blood groups to a universal blood group “O”. As we know that “O” group would suite to everybody irrespective of our own blood group. Very interesting innovation, i think these guys certainly deserve for a “nobel” kind of prize.