Reasons not to grant db_owner
DBO can do some nasty stuff:-

•  CHANGE RECOVERY MODEL
•  BACKUP DATABASE
•  ALTER ANY SYMMETRIC KEY
• TURN ON/OFF AUTOSHRINK
•  ADD FILE
• AUTO_UPDATE_STATISTICS
• SET RECURSIVE_TRIGGERS
• DROP DATABASE
• SET OFFLINE

If you must create database objects, view showplans/dynamic tables or more but do not want to give away ‘the keys to the city’, consider creating a custom role.

— Avoid using the db_owner role by creating a new role with enhanced permissions

use somedatabase
go

CREATE ROLE [enhanced_role]  AUTHORIZATION [dbo]
GO

Some Notes:

AUTHORIZATION [dbo] means the role is owned by dbo.   If no user is specified, the role will be owned by the user that executes CREATE ROLE.
There are a lot more possibilities for beefing up this user.

GRANT ALTER ANY APPLICATION ROLE TO [enhanced_role]
GRANT ALTER ANY ASSEMBLY TO [enhanced_role]
GRANT ALTER ANY DATABASE DDL TRIGGER TO [enhanced_role]
GRANT ALTER ANY DATASPACE TO [enhanced_role]
GRANT ALTER ANY FULLTEXT CATALOG TO [enhanced_role]
GRANT ALTER ANY MESSAGE TYPE TO [enhanced_role]
GRANT CREATE AGGREGATE TO [enhanced_role]
GRANT CREATE ASSEMBLY TO [enhanced_role]
GRANT CREATE DATABASE DDL EVENT NOTIFICATION TO [enhanced_role]
GRANT CREATE DEFAULT TO [enhanced_role]
GRANT CREATE FULLTEXT CATALOG TO [enhanced_role]
GRANT CREATE FUNCTION TO [enhanced_role]
GRANT CREATE ROLE TO [enhanced_role]
GRANT CREATE SERVICE TO [enhanced_role]
GRANT CREATE SYNONYM TO [enhanced_role]
GRANT CREATE XML SCHEMA COLLECTION TO [enhanced_role]
GRANT VIEW DEFINITION TO [enhanced_role]