This storyline has a precious history in professional wrestling, especially in the days of weekly televised shows.
Nash and Hall. The Radicalz. The Alliance. The revised ECW. The Nexus. And, multiple occasions where Raw and SmackDown superstars "invaded" the other show during the "Brand Extension" years.
An invasion storyline sells. Not just merchandise—although the sales of NWO and Nexus shirts must be mentioned—but it helps sell pay-per-views and raises the viewership of the televised shows as well. It is one of the most repeated "big" storyline arcs in the business.
It is a simple storyline to recreate. All you need is a group of "outsiders" and a simple but well-designed shirt. And you can get away with that for a few weeks, until you need to add more substance to the invasion.
The invasion should have a charismatic leader who is able to deliver on the mic. It can do well to include a manager, whose work is to ensure that all the "invaders" are on the same page. And, last but not least, it needs a motive.
The invaders have to be people that can be easily identified as a group. In the case of Hall and Nash, or the Radicalz, they invaders were clearly people who were associated with a rival company. Similarly, former WCW and ECW superstars were easily identified by the audience in their storylines. The Nexus invasion occured immediately after NXT's first season concluded, when Wade Barrett had earned himself a WWE Championship match.
Now, the revamped NXT is going strong once more. With Dusty Rhodes at the helm and a newly crowned NXT Champion in Seth Rollins, the "other brand" of WWE television is at seeking to reach—and hopefully surpass—the level that WWE's version of ECW reached. But, it is still just a niche show in the company's programming and is not featured on the monthly pay-per-views. In fact, it is even hard to spot on WWE's website.
In 2012, CM Punk, John Cena, The Rock and Brock Lesnar have been the main pillars of the "big" storylines, and will certainly continue to do so until WrestleMania 29; they will likely be assisted by Undertaker's Streak and Triple H's corporate dealings. The best that anybody else can hope for is to be featured on the World Heavyweight Championship side of things.
But, after WrestleMania, it is open season once again. WWE has a tradition of "Summer Storylines"—a concept that brings in something from left field to refresh the scene and lead to new feuds as April begins to fade.
NXT will probably have had a few more champions by then. The roster will be more ready for the bigger stage. Hopefully, the NXT Championship will get a chance to open WrestleMania 29, bringing the show enough spotlight to stand on its own. And then, we can get down to business.
Unlike the Nexus, the NXT Invasion will continue to have its own show (NXT), which might occasionally be invaded by the stars of the main roster for retribution. This will ensure that more people tune in to watch the third-tier show, to get some insight into the invaders and to follow the storylines in more detail. And, like the ECW, NXT might one day have the WWE Championship defended against the NXT Champion in its own yard (actually, I'm not too sure that the hall in Full Sail University can accommodate such a spectacle.)
An invasion always brings with it the chance for new alliances and changes in allegiance in the roster it is invading. Disgruntled midcard and lower card superstars in the main roster might decide to join hands with the NXT Invasion. And, managers (Paul Heyman and/or Vickie Guerrero) might align themselves with the Invasion, in order to achieve their own selfish motives.
Such an invasion would allow the WWE to build fresh feuds and push new superstars into the limelight. It would also be a useful storyline tool to upgrade NXT talent to the main roster. It would help draw fans—and, as mentioned, their monetary investment—and help balance the playing field after whatever WrestleMania brings.
But, the most important reason is that an invasion storyline works. It requires minimum creative effort, at least in the first few weeks—some fans will argue that WWE Creative has the absolute minimum—and the impact is immediate. It is a storyline that brings new superstars into the limelight, without messing about with storylines that make no sense. And, as long as the invading group isn't "overcome" by a single superstar, the storyline can directly lead to the invaders having their individual feuds in the main roster, saving time which might have been spent making random tag teams—or worse, having superstars that appear only in random backstage segments.