The 'new' ECW was definitely the prototype for NXT. It was meant as a jumping point from OVW to Raw/SmackDown, but foolishly coasted on the ECW name. Had it been called NXT from day one, it would likely have been much better received. They even got to have their championship matches on PPV cards, wich was great for the up and coming talent of the time.
If you were able to look past the fact it wasn't Paul Heyman's ECW, there was some good shit on there. It gave rise to CM Punk. John Morrison had an awesome run. It put guys like Big Show in a spot where they actually worked well. Even Shamus had a great run back then. Had the format not been cancelled, they wouldn't have done that reality show version of NXT and Daniel Bryan's debut would have been in actual wrestling matches.
Contrary to some popular beliefs, Vince actually loves competition. When Raw and SmackDown were competing brands, both shows benefited (SmackDown moreso), especially when they would have a draft. USA Network was the party that wasn't pleased with the fact the A show was becoming the B show, and that's what ultimately killed the brand split.
It made it feel more like a real sport too, having divisions that could compete with each other, but mostly separated. And it made throw-away PPVs like Survivor Series a lot more special. When guys that are on a good roll, but on separate shows, meet for the first time, its always a huge draw.
The original Brand Split happened when WWE had a HUGE roster, though. This time, any brand split would need to be treated differently. If they truly take it as an opportunity it might actually be better than the last one.
The IMMEDIATE benefit is everyone gets a lighter schedule. This addresses their current injury problem without straight up pulling people off TV for lenthy periods of time. Superstars can get a lot more rest and rehab between bouts.
With superstars working lighter schedules (and probably being in better shape for it), they will also be capable of putting on longer matches for TV tapings. So even though the rosters may be thing, we could get better/longer matches as a result to fill up the air time. And this is going to mean the midcard will be asked to carry a lot more of that load.
The natural result of the midcard getting a better showing is that the upper card will be less overexposed (and feel more special), and the midcard championships will have a lot more value.
The other benefit of a lighter schedule is the talent can actually go down to Florida more often to work with NXT talent. You could have a lot of special guests pop in and end up in matches that help get new guys over, or even start feuds that will help push them to the main roster later.
Filling 5 hours of TV might also be simpler than expected since the McMahon drama is back on TV and that tends to eat up a lot of TV time that could be spent showcasing superstars.
The biggest benefit for a Brand Split though is simple. It's something new(ish). And that's pretty much the one thing fans are asking for.