The downside of the random key generation approach is that you must back up each new key before it is used to encrypt data. If you do not, then the protected data will not be able to be decrypted if the key store fails.
Comparatively, derived keys offer some significant practical advantages. Since the secret changes only rarely, backups are infrequently required and the need for the whole create-activate-name-deactivate sequence (other than authorization) is removed. Multiple key servers can be created from a single backup and are guaranteed to derive the same keys from the same inputs, since the original seed material is reused, without requiring any real-time replication or synchronization. There is also no risk of losing keys: if an application loses a derived key, it can be re-derived as easily as generating it in the first place.