Everything definitely should NOT be encrypted. The US military learned long ago that "overprotecting" classified material was as bad as no protection.

The reason? Human nature.

When all material had to be handled as "Top Secret", including information that is unimportant or irrelevant, people tend to assume that none of the material is really that important.

Encrypt everything and you will begin to see encryption keys written down on post-it notes hanging off monitors.

Also, there is the issue of cost in both time and money.

It may be easiest for a DBA if everything is encrypted, but that doesn't mean it is best for the organization.