The idea of the filler material is to hold the liquid and feed it to the atomizer as needed.
The bridge-looking part of the atomizer touches the filler and provides the path for the liquid down to the atomizer.
If the filler material is feeding the atomizer too slowly you will start to get a burnt taste if you take too many drags in a row.
If the filler material is not holding the juice well enough, too much juice will flow out and flood the atomizer.
The general problem with the filler material that comes with the cartridges is that it wicks too slowly.
Another problem is that it doesn't wick well enough and often leaves a fair amount of juice in the bottom unused.
That is why people frequent the cartridge mods subforum and experiment with different ways to improve this process...
http://www.e-cigarette-forum.com/forum/cartridge-mods/
But anytime you find a filler material that wicks better or more quickly, you face the potential for it not holding the liquid as well.
Then you get leaking and flooding problems.
It is a balancing act, and many people eventually get tired of playing the game...
--Some people find a filler material that works for them
--Some people find a cartridge modification that works for them
--Some people turn to cartomizers
--Some people turn to dripping
Here is a poll that may interest you...
http://www.e-cigarette-forum.com/forum/polls/92564-do-you-use-cartridge-cartomizer-drip.html