Cause i have an empty skillslot 
Imagine deep freeze or 10% extra chc !
Or imagine the lightning hydrae using some of the electrocute runes -now that would be cool wouldnt it?
I mean that would be nice, but ultimately I wouldn’t care too much. It needs a 2-3x damage boost to be viable which a runed FN certainly won’t do.
If you have an empty slot on your bar you are probably using a speedfarming build, so you can probably fit a winter flurry sourse in your build since the only other useful offhands are channeling offhands that are redundant farming t16.
As far as I know winter flurry DOES proc the frost nova runes, so you can get the extra damage modifier if you want it.