I'm a 63 frost mage currently ( http://www.wowarmory.com/character-sheet.xml?r=Ysera&n=Quantos) and I love AoE'ing and am now half decent at it. For me the most important parts of AoE'ing are the pull, and the snare (frost nova'ing everything in a tight group). I have never tried to work my WE into this since I don't think it would help you pull more mobs. Its extra FN might be helpfull after a good pull though.
I have found that when I was lvl 60 AoEing some of the lvl 61-62 buzzards would require 2 runs for a total of 3-4 Blizards(IB, mount, do the pull and round up 10 or so, CoC them, IB again, FN, run, Bliz, Bliz and repeat, wand any survivors) and if any resist my second FN, I had to decide between CS or poping my WE for his FN or running away. I found that if some resist the FN you need split second reaction time or they will be too spread out, and for this reason I'd say that unless you already have your WE out the extra .5 second to pop your WE and use his FN would be bad.
I usually just use my WE for adds and elite's, as the other posters have said too. Though if you pop your WE before the pull you could use his ranges FN for the 1st snare giving you more range and time. I think I'll go try that when I get home tonight :P