Nightshade said:
By doing this they don't get all the vitamins they need and this can lead to calcium deficiency plus many other deficiencies.
seriously tho, while i am all in favor of feeding home prepared diets (raw or cooked), it's never a good idea to just jump into it without research, and there are plenty of good books available on the topic.
as long as you follow a few basic rules, there will be no deficiencies at all.