I think the problem with Clause 4 is that it's almost been treated an end in itself, and has been treated that way a few times, eg 1945-1951 when I think basically everything was nationalised pretty much just for the sake of it. If that happens then you have a service that isn't necessarily any better-run than before and indeed could be worse off, although circumstances in the middle of last century were rather tough to use as a way of judging the idea.
It seems to me that nationalisation as a principle can work if there's no reason to expect that "true" market forces could ever come into play. Take the railways -- exactly how are you supposed to have competition driving prices down and standards up if the only route from Leeds to London is by the East Coast main line service, currently run by Virgin and Virgin only? My experiences travelling up and down the line have been generally pretty nice -- a reliable and fairly quick service, but it feels pretty much the same as when National Express ran it.
Nationalisation could in principle make the railways better, more efficient and more coherent across services. Whether it actually will make a difference or not I can't say; having missed the last time the railways were nationalised I don't know what they were like then (pretty awful, I'm sure is the usual impression) -- but if a better job were done this time round then it might well work out well. Or not.
I don't believe in nationalisation as something to be aspired to, anyway. It's hard to believe that Labour's future is served well by resurrecting a debate that really should have been put to bed 20 years ago.