Too many nether portals in one area break the hostile mob cap

By building a large array of nether portals, one player can generate pigmen beyond the hostile mob cap. This can prevent normal spawning server-wide. I'm not sure how to test the limits of this, but it can affect at least several simultaneous, disparate players.

By operating one continuously, a small group can shut down mob spawning on a server, in a non-obvious way, until an admin realizes what's happening and chooses to step in. A single person can essentially put a server into a quasi-peaceful mode this way.

This can be used as a form of griefing, or be exploited by two people to explore and make safe otherwise dangerous areas without fear of hostile mobs.

Because it's not obvious to the operator that this is occurring, nor to other players the cause of the paucity of hostiles, it can cause inadvertent problems.

I'd suggest that spawns from nether portals be limited to one portal per chunk.