Minecraft wiki entry for Things_not_to_do, associated text: When in a dungeon, mineshaft or Nether fortress, don't try to break any spawners, with the exception of the Sliverfish spawner found in the Stronghold. A spawner drops only 50 experience points when broken, but a working spawner can be used for mob farms to gain more experience and rare drops. The spawner should simply be disabled by placing a torch on one side, except for blaze spawners, which require many torches within a couple of blocks of it. An exception may be the stronghold since it may be annoying to try to set up a silverfish farm and the player may wish to just get experience other ways.