What am I missing

I am going to start this with a confession, I have read Private Eye since my teenage years. I am not so blind a fan to not realise that reading it requires a reasonably high amount of cynicism, more so than for most other quality newsprint except perhaps the comment pages of the strongly "sided" newspapers.
Of the many journalistic certainties in life like "British newspapers can't cover science to save their lives" it is a true universally acknowledged that Private Eye is against PPP especially PFI. With the collapse of Metronet the issue is in the foreground again and it has reminded me to go looking into what the advantages are and I am obviously missing something as I can't work out what they are. I can see the argument that it keeps the money away from the government's debt column. But is it worth it? As pointed out given some of my reading material I don't hear much about anything but those that fail, but all the stories are of overspend, delay, corners being cut etc. Given that the government is considered a safe bet for money lenders this seams very much like taking out a very expensive hire purchase agreement when you could just use your overdraft which is much cheaper.
Of course there is also the issue that risk should be transferred, again however the stories show that in practice this isn't the case. But if the government raises the money itself and then contracts the private sector to build and even maintain the asset, if the service is bad or expensive then there is much less of a lock in to prevent them changing providers.
If the idea was bought up as new today with accurate numbers like we know have in hindsight, would any government or council be able to sell it to the voters, so why do we carry on.