Simply put, I installed the beta just now and installed a few addons to add some new mobs to the game, I attempted to enable experimental gameplay and not have it enabled. In both instances, no spawn eggs appeared, and no added mobs were added in the specified biomes (nether, beach, desert). I wandered around in creative for a good 10 minutes, both at day and night, and nothing. Are 1.8 addons disabled in the current beta, or are they broken?