One of the other things you could get by identifying object types by object id is support for run time creation. End users don't give a hoot about system wide unique text primary keys, so you will have to come up with some scheme to generate them (reinvent sequences), or teach end users about the rules of creating primary keys. If integer object_id primary keys are used to identify object types, the text identifier object_type can be dropped (saving 30 bytes per row in acs_object_types, all the tables which refer to object types, and their indexes)
Of course there is still a need for programmers to identify some object types by name. We already have a scheme for this, acs_magic_objects, a mechanism for assigining unique text labels to object ids.
The upgrade script would be... a challenge.
