I don't think the majority is really getting what is going on here, so I'm going to go ahead and try and explain it all.
Wade Barrett won NXT Season 1 and was guaranteed a spot on the main roster. Unlike Daniel Bryan or even Heath Slater, he never got a marquee victory on NXT over a pro or anything. He was ready, but he hadn't proved that he could beat the top talent or be anywhere close. Thus, enter the Nexus.
Barrett manipulated his friends and told them that their purpose was to pay WWE back for treating them poorly, and this got them on board with ransacking Raw and reeking havoc until they got contracts. Why? Having contracts helped Wade Barrett have help from his friends as he tried to move up the WWE ladder.
Unfortunately, his title shot was removed because of the attacks, but that was solved as Nexus continued to raise hell. Despite what was being said, the plan was always about getting Barrett to the top. The thing is, Barrett set the whole thing up so it was always about him and the others were his whipping boys.
As time went on, he made it known that it was all about him and he started to treat the Nexus and their newest member, John Cena, continually poorly. Otunga said something about it and pissed Barrett off, but for a while, it was a stalemate.
Now though, Barrett showed that he can't get the title and Cena finally took out Barrett. Thus, Nexus isn't sure where to go. Enter CM Punk.
With a group that IS bonded together without Barrett, they need a new direction. They still don't like Cena who makes fun of them and in walks the master manipulator. Punk tells them of his hatred for Cena and his hypocrisy and Nexus buys into it. They like Punk and he seems like he'll treat them better than Barrett. Thus, they have a new leader and Punk has a group to help him out as he tries to do what Barret and Nexus could not.
So why should it work with Punk and not Barrett? For one thing, Barrett was a simple bully while Punk can really get in your head. Punk's psychological warfare are what could take down Cena more than brute stretch or mat ability. Punk's psychology along with Nexus' backing make a lethal combination, and it could be more lethal than when Nexus began.
As for questions of "Punk got beat up initially by Nexus", the initial attack was to make an impact, and it did. Never again did Nexus target Punk though. They weren't enemies and as you can see, Punk is really good at manipulating people. Hell, he kinda manipulated Sheamus who doesn't like much of anyone. Explaning a common enemy and a plan will certainly put to rest a grudge (if any) between Punk and Nexus.
This actually makes a lot of sense when you think about it. Trust me, it works and does not have the plot holes a lot are saying. Take a step back and read my words. They will enlighten.
I have high hopes for this move, though I don't know that Nexus is entirely necessary. However, being a heel means using any resource possible and necessary, so if Nexus is just sitting there, you'd be stupid not to use them if you're Punk. Let's just see how this progresses. So far it's excellent.