One of the NFC victories against the 3-4 was SD-Atl.
Atlanta played the last two years the 3-4 as well. The Oline and Vick trained against it and knows it quite well. Therefore SD lost the advantage fo playing an unknown system.
Overall the reinforcement of the 5 yard rule and the flare-up of short passes means you have to blitz and do it fast. Some teams do it with Safties and CBs and the 3-4 brings the advantage, that you can blitz from 4 different LB spots regulary. Disguise by system.