DBMS_RANDOM considered dangerous?
The reason why DBMS_RANDOM should not be granted to PUBLIC, when using it for crypto key generation, is that an attacker could use it to determine seed values and/or patterns in the key generation, that could be used to determine the key the data is encrypted with. This is why it could lead to compromise of the encrypted data. It certainly is not an easy attack, but it is possible for someone with enough processing power.
DBMS_RANDOM should not be used for crypto because it is too predictable. For crypto key generation, only a secure random function should be used. These functions attempt to get as random as possible by measuring things like white noise and producing values off of it.