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One wonders then what was ever the point of Schedule 4 of PoFA if they can just initiate proceedings against the keeper and succeed in court because on the balance of probabilities the keeper would have been the driver.

Because if the keeper was not the driver they can still hold them liable if they comply,  S4 was never about providing some form of a technical defence to the driver which, to be honest, is what you seem to be seeking.  Keepers were taken to court before PoFA anyway.

IMO, it's much more fundamental.

If a creditor reasonably believes that the keeper was the driver when they start to pursue the alleged debt then it's an abuse of process to issue a spurious PCN addressed to the keeper dressed up in PoFA-like references, but not PoFA itself, to embroil the keeper in their claim. Even worse, possibly to go to IAS with a claim against the keeper and then to have a miraculous epiphany on the road to court such that their claim is transformed from one against the driver to one against the keeper-cum-driver especially if they offer any evidence obtained from the keeper post issuing their PCN.

Clean hands, my a**e. 

Their logic here is usually that if the keeper was not the driver, he would have said so, as this would strengthen his case considerably, and so the fact he has not implies that he was in fact the driver.

There could be a good reason why the keeper would not identify the driver even if it were not them.

The underlying contract was formed by the driver, whoever that may be.

The driver disputes the existence of an underlying contract.
 
The parking company are refusing to provide proof that they had the right to enter into a contract with the driver, but that argument was put to one side when it became known that they don't have the right which they claim to pursue the keeper for the alleged debt.
 
I.e. the hope is that the issue of the existence of any debt in the first place becomes irrelevant because they have nobody to pursue anyway.