Eating rabbit is not harmful - there's no reason to avoid it. Its just that eating rabbit alone is not enough in the long term. According to this article:
http://www.medbio.info/Horn/Time%201-2/vilhjalmur_stefansson1.htm
Your body can only process at most 1600 calories of protein a day, which is a starvation level ration for most people.

So if all you eat is rabbit you will eventually starve to death.

If you don't eat ANYTHING, you will starve to death much faster.