Prosecuting directly meant there was no outside eyes on a lot of the cases, no questions about if it was actually in the public interest to prosecute, or external assessment of the evidence before pushing for plea deals.

It's not a foolproof system, but I think most of the cases you'd find about 'dodgy evidence' would also mention a 'overly close relationship between police and prosecutors' - because fully independent prosecutors should be rejecting cases with dodgy evidence. The police can get a bit grumpy with CPS for being quite strict at times - but it holds them to work their next case properly.

The PO clearly managed to convince the CPS in a number of cases, but had they had to do that for all there would have been a lot less cases prosecuted in total, and I think there would have been questions about their system a lot sooner.

It might not have avoided the problem, but it would have been much needed additional oversight into what they were doing.