The Time variable in FallingSand entities contains the time of how long the Entity exists. In Minecraft itself it is an integer but the NBT TAG type is only a byte.
Wrong saving: Technically you could keep a FallingSand Entity forever alive (if you can hold it in the air) by just reloading the world, the time TAG would be maximum 127 but FallingSand entites get only deleted when the time is greater than 600, so this part of the code is never going to be executed.
No full control of FallingSand entities when spawning with summon command: Because no matter what number you will put in for Time, the maximum is 127. This is important if you want for example an entity to appear for a certain amount of time and disappear. An example of why this technique is so important http://www.minecraftforum.net/topic/2003346-animated-walking-man-13w39b/
It isn't needed to change the acutual TAG type in the save files, except you want to fix the first mentioned problem. Interpreting the right type out of the summon command is more important (to me ). So numbers greater than 127 take actual effect.