True. The centre-ground of the party largely treated Corbyn like crap when he got in and never particularly gave him a chance, and Corbyn's performance has largely proven them (and many of us) wrong, but I'd say he should be the bigger man on this one and try to unite everyone around him; so far, to his credit, he's been doing that, welcoming figures who resigned etc back into the shadow cabinet.
For as much as guys like Campbell, Mandelson etc may be snakes, they've had their uses in the past and I'd say it's much better for Corbyn to have them on his side than against him. They don't even necessarily need to be in incredibly prominent, senior roles as such, but if they're willing to play a part then they can be included and could be useful. Hell, if nothing else, Corbyn can rip the piss out of the Tories by claiming he's a unifying figure of strong and stable government, someone who's able to unite his party and put differences aside, while they are the coalition of chaos; welcoming in a corrosive influence such as the DUP to government, and shafting the country over to suit the whims of their party more than once.
Plus, Attlee was known as a consensus compromise-type politician, someone who could work with all wings of the party, and he's the best Labour PM (and perhaps the best PM) the country's ever had.