Advantages to both sides, although in the long scheme of things, I prefer one world title.
I like having one world title and having everyone compete for it to become the one and only 'best.' You also don't have to worry about two people being on different shows if you want them to feud or wrestle, if everyone is all on one brand.
On the other hand, the brand extension did have it's advantages. More people get a main event push, and you can push twice as many feuds. It also stops the main event scene from just revolving around the same 4-5 guys for several years. If there had only been one world title since 2002, the main event would constantly revolve around Cena and Orton and Batista and Edge and Triple H switching the title back and forth with little exception. The brand split prevents this (kinda.) But something I did always dislike about it was the fact that it makes winning the Rumble mean a lot less. Winning the Rumble used to mean getting a direct spot in the main event of WM against the one world champion. With two world titles, that means you can always find a way to get a world title match at WM even if you fail to win the Rumble. To make it worse, the world title match with the non-Rumble winner very often (more often than not since the brand split, actually) gets the main event slot over the other title match.
My feeling is that the brand split has run it's course, as I always suspected it would. I never thought it'd be some permanent thing. It's all just one show now and having two world champions both saying they're the 'best' in the company doesn't make a whole lot of sense. I think it would make sense to go ahead and combine the belts at this point, but of course they probably can't do that due to Mattel having creative control in their contract with WWE. So, perhaps the best solution is to make it actually feel like a brand extension again. That doesn't just mean splitting the rosters, but also bringing back brand split PPVs, a concept which I actually enjoyed.