yeah you can do more than one ip. so let's say I'm running a webserver on 192.168.1.1 and a mail server on 192.168.1.2 and i need to take the webserver down for a little while. i could have the mail server take 192.168.1.1 but keep it's 192.168.1.2 and just have a dummy webpage that explains it being down. this way i don't need another nic.
or it can be used to sniff someone elses traffic through a switch, but arpspoof is a cleaner method of doing this.
[edit:moving to networking]