I agree with Coobie in that everyone has a tale about this and that that works for them.  Personally I have never used any additives in my diesel and never had any issue with gelling or slime in the tank.  Some folks do however have problems with both issues.  I know when I worked in Northern Alberta Canada we had -40 temps much of the time and none of the equipment got special additives other than winter blend fuel.  If the winter fuel blend is done right, you shouldn't have any issue with it.  NOW, how do you know it is good for low temps. NO WAY for the average Joe to know so now we have special additives to prevent certain things (at least advertised properties) that get folks to buying them by urban myths and tales and the low life suppliers that sell us summer blend in the winter because they still have some in stock.  I think this issue is what causes all the tales of needing additive started. 
Whether or not they are required is unprovable since lots of users can say that they have used XXXXXX for 10-40 year and never had a problem.  How can we prove that those users would not have had a problem if they didn't use the snake oil?  We cant, which is why the products continue to be used.
I do know that one additive works for gasoline storage and that is STABIL.  I know first hand what not putting it in my lawnmower gas did vs putting it in every year since.