That only works if you use a hub instead of a switch, or if the traffic is broadcast, or if you somehow convince the switch to give your port all traffic. It's easiest to just use two NICs bridged in one PC as a MITM.
You can also easily run an ARP spoofing attack, and not just to sniff but also to manipulate. Done that quite often back in my school time, oh the glorious days when web traffic was mostly plain HTTP.
There were entire command lists going around specifying what exactly one needed to replace, say, the logo of an online newspaper with a picture of a dong.
Even an el cheap basic managed switch will do port mirroring