You would need something more than just the nMP and a switch to get utility out of two ports.
If less than two other systems trying to connect to then a switch aggregating and/or VLAN switch probably isn't needed.
Generally the second port is useful for some internal LAN only traffic. If you don't often have 1 Gb/s worth of that kind of traffic then it not going to get much leverage out of the second port. Not particularly material was wrapping that traffic in a aggregating/trunk/bundle , VLAN, and any other "connection mechanism" if the raw bandwidth need isn't there.
The nMP doesn't have lots of bulk storage capacity. If going to a NAS solution for that then the second port could be assigned to a independent local NAS data traffic network. Either just the NAS box and the nMP or perhaps another swtich, the NAS box , and multiple Macs.
Two examples:
nMP ( port 1: currrent router port 2: NAS box )
NAS box ( port 1 : nMP )
nMP ( port 1: current router port 2 : storage_switch )
NAS box ( port 1 : storage switch )
MBP ( port 1 : storage switch ) [ wifi on router's wifi network ]
Need to do minor configuration of "storage network" so not using same LAN addressing, but no managed switches anywhere in sight. Keeping your storage network clear of general internet traffic ( Netflix , VOIP , streaming audio , flash ad videos, etc. ) should deliver more consistent and better bandwidth to the storage data.
Similarly the "NAS box" could just be another Mac that is tasked with file sharing duties. (e.g., an old MP. ).
You can step up to a single switch with VLAN and/or aggregating capabilities later if get into zone of more than a handful of devices on your storage network.
But unless have a 3 or more nodes in a highly trafficked network then there isn't much leverage for the 2nd port.
