There are use cases for using the baremetal platform and having the baremetal capability while disabling MachinePI (for example to use CAPI ).
Currently, there are few validations preventing this in the installer and in openshift/api, these validations exists because CBO will crash if the Machine CRD doesn't exist on the cluster.
CBO is usable on many platforms, and depends on MAPI on all of them, when it only really needs it for baremetal IPI.

Feature goal (what are we trying to solve here?)

Allow using the baremetal platform and having the baremetal capability while disabling MachinePI{}

Does it need documentation support?

yes, we should update the docs to say this configuration is valid.

Feature origin (who asked for this feature?)

• A Customer asked for it

• • Name of the customer(s): Orange telco cloud (this is required for enabling CAPI to install multi node OCP)
  • How many customers asked for it? other costumers can benefit from this
  • Can we have a follow-up meeting with the customer(s)? sure.

• Internal request

• • Who asked for it? ercohen, zabitter, joelspeed  

• Catching up with OpenShift

Reasoning (why it’s important?)

• Please describe why this feature is important
  There are use cases for using the baremetal platform and having the baremetal capability while disabling MachinePI (for example to use CAPI ).
• How does this feature help the product?
  it makes more sense to enable CBO to work without MAPI }}{{since CBO is usable on many platforms, and it has the same dependence on MAPI on all of them, when it only really needs it for baremetal IPI

Overall this should allow users to install OCP with baremetal platform using ABI or assisted installer while disabling the MachineAPI capability.
Here is an example patch for configuring this when installing with assisted installer:

curl --request PATCH --header "Content-Type: application/json"     --data '"{\"capabilities\": {\"baselineCapabilitySet\": \"None\", \"additionalEnabledCapabilities\": [\"openshift-samples\", \"marketplace\", \"Console\", \"baremetal\", \"Insights\", \"Storage\", \"NodeTuning\", \"CSISnapshot\", \"OperatorLifecycleManager\", \"Ingress\"]}}"' "http://$ASSISTED_SERVICE_IP:$ASSISTED_SERVICE_PORT/api/assisted-install/v2/clusters/$CLUSTER_ID/install-config" 

"