How important is -80C?

[Chris Jones]

We're looking into replacing one of two -80 degree freezers, and the 
question came up as to whether we should bother. Currently most of the 
contents are bacterial and plasmid stocks as well as some isotopes. It 
seems to me that these would fare no worse in a -20 degree box, but I 
wondered what experience others have had. What samples "must" be stored 
at -80C?

Thanks,

Chris Jones