As some may say this is "Works as Intended", I would disagree. Summon should still work on a server, even if the server.properties says spawning mobs is false. The server property for these instances should only affect natural spawning - not mob spawners or commands. This is important to certain mechanics of custom maps that rely on villagers and animals spawning in a custom map. And I believe those properties were created specifically for survival worlds.

The reason I believe this is actually not intended is that you can still spawn hostile mobs with summon if spawn-monsters=false but not animals or villagers.

To reproduce, run a server and change server.properties to:

    spawn-npcs=false
    spawn-animals=false
    spawn-monsters=false

Now hop on the server and place 3 commands blocks with these commands:

   /summon Villager ~ ~1 ~
   /summon Wolf ~ ~1 ~
   /summon Zombie ~ ~1 ~

With difficulty on non-peaceful, you can see the Zombie summons fine. But the Villager and Wolf do not stay in the world.

For custom maps, commands should override any server.properties as it does with hostile mobs.