The code which does normal animal/monster spawning (MCP naming class SpawnerAnimals.findChunksForSpawning(), or func_77192_a()), uses a single spawn point fetched as worldServer.getSpawnPoint() to keep a volume around it clear from spawning monsters.
However, sleeping on bed or using command /spawnpoint does not change that location (tested by console-logging the fetched location in the spawning method). It would also seem theoretically impossible to handle the spawnpoints of multiple players (on multiplayer server) with that code. (I think that location is the initial spawnpoint for single-player.)
Keeping an apparently "random" single location in the world clear from monster spawning, as it is doing now, seems pointless. More appropriate operation would be either ignore that location completely, or change it so that it considers the effective/current spawnpoint of any and all players.
MC-10303, where the current functionality causes mobs to not spawn near that spawnpoint when using superflat gametype.