Dryhopping during fermentation means that those awesome hop aromas will blow off with the co2 during fermentation. I dryhop in the primary, but I wait until 5 days before packaging before dryhopping. Once fermentation is complete, and the beer is clear, I dryhop and wait 5 days and then package.
Oils from the dryhops will cling to yeast, so dryhopping during active fermentation is a bad idea for those two reasons. (The hops oils will cling to the yeast and fall out when the yeast flocculates, plus any hops aromas will blow out with the co2).