The main point of Kerberos is authentication - it allows networked entities to prove their identity. In order for this to work, each entity needs to have a unique name (in Kerberos jargon, "principal name").
You won't want to share one keytab among all machines, because a keytab contains the secret key of a specific Kerberos principal name. Sharing a keytab would mean you are sharing that keytab's identity among all machines. The machines would all use Kerberos to declare that they are the same machinename, and Active Directory will not be able to tell the machines apart.
Consider an analogy: trying to share one keytab among multiple machines is similar to trying to share one Kerberos principal name among multiple users. You can instruct five people to all use the same username and password... and they can log in... but the system they log into will not realize that there are five separate people. The system would see multiple connections to one single account, since all the connections are using one single Kerberos principal name.