Agree with maximegir above (point given), but just in case you're thinking, well, that's okay, when I cook the chicken any bacteria present will be killed, there's another factor to consider. Bacteria will have been present in the chicken before it was frozen, all living things have bacteria; once it's frozen, the bacteria are arrested by the freezing process. Defrosting in a refrigerator obviously means it's cold enough still for most bacteria to be unable to function normally, but if you defrost at room temperature, parts of the chicken will be warm enough for bacteria to start doing their thing. When bacteria are active, they produce by products which, in some cases, are toxic to us, and these toxins are not inactivated or destroyed by the cooking process. So, in some cases, illness might be caused by eating chicken defrosted at room temperature and then cooked, even if its cooked properly.