Another use of dual nics is for failover incase one switch you're connected to fails. Yet another use for dual nics is for backups. Many large providers have a dedicated backup network so that all network traffic while doing file backups on your network are over a seperate network so it doesn't effect bandwidth on the main link. Yet another reason is to bond the two connections using etherchannel and you can get a 2G network link instead of a 1G link. I can go on and on and on with options, we have at my office atleast 15 different reasons in use for needing multiple nic's.
Brian