If you unify the midcard titles, it's not long before you unify the women's titles and the world titles as well. Personally, I'd love it if there were only one championship amongst the divas as that division has about the same focus as the tag teams do, and it would allow them to focus less on it in a way that could maximize the product (ie, instead of having to require the use of people like the Bella twins and Rosa for filler in a tag match, all the divas could be interpromotional, therefore you could just focus on the ones that the fans care about). Anyway, I digress, this isn't about the women's division.
I think having two world champions is a good thing, at least for now. It offers flexibility. You can have a heel and a face, offering the fans two different types of challenges. You can have a long reign with one belt and the other one changes hands a bit more often so that fans don't get tired, but it doesn't seem like the hot potato scenario. You can boost someone like Jack Swagger up to the main event rather than just cycling through the "safe options". The same goes for having two midcard titles. If you've got two world champions, it's beneficial to have two midcard champions as the "stepping stones" on their respective brands. Plus, if they continue the Bragging Rights concept, you've automatically got a built in adversary concept. First time around, we had the IC champ John Morrison from Smackdown against the US champ the Miz from Raw. To coincide with the benefits of two world champs, the two midcard champs does exactly the same, but needless to say for lower card guys.
Then there's the issue of "what are they fighting for?" When the divas on Smackdown didn't have a title, everyone questioned it, saying it felt like they competed for nothing, so they created a second title to be placed on that brand (which of course could be merged with the other and dual-branded), and you interpromote the tag titles, but there's a limitation. The tag title scenario involves 4 wrestlers going between the two shows. The divas situation would involve what, 18 divas, max? If you take out the Bella twins (as they do nothing but walk guest hosts to the ring now), Rosa (is lucky to appear backstage or standing next to Ryder), Katie (when's the last time we saw her?), Savannah (ring announcer, doesn't wrestle), and Serena (accompanies Punk, doesn't wrestle yet), you've got 13 divas that are actually participating, if you count Jillian. Not too hard to keep them going from show to show, especially if you don't have any plans on using some of them because you're focused on one primary feud. But you can't do this with the men in the WWE, having a midcard champion go between both shows and have ANYBODY be a potential challenger to his title. That's far too much, and then everyone else in the process is all just having meaningless matches or repetitive "a series of matches means we're in a feud and I hate you" storylines, which gets boring. Having two titles gives you two solid feuds revolving around the titles and you don't need to worry about spreading yourself too thin like you are with the divas and tag divisions because you've got waaay more than enough people to pick from.
Plus, you know they'd keep the IC title, and the Miz is awesome and McIntyre isn't exactly blowing up the spectacular scale, so that's another reason not to merge them yet lol