Planetside directly benefits from having more players on at a single time. DCUO doesn't.
Now that it's F2P and it has a cross faction queue system for PvP, I agree that it doesn't really matter in DCUO. It was a whole different matter at launch though when the game required a $60 purchase, a sub, and pop was bleeding out causing queue times to take forever and making it impossible to fill out a raid group. Cross platform servers would have benefited both platforms during that first year.
It just seems to me that even though they've made some good progress on the front with DCUO, there's still several issues that preclude cross platform servers from being a sure thing for Planetside 2. Like how are they gonna handle the controller differences between both platforms? It's not that big of a deal in a third person action game designed from the ground up to the limitations of a controller, but for a PC FPS, M+KB will clearly have an advantage over a controller. Some will argue that letting M+KB on the console port will solve the issue, but I'd argue most people on the console side won't bother plugging in a M+KB and letting in people with a M+KB advantage into the console servers will sour the first impression for those users.
There's also the monetary side of the equation. It's been 2+ years since DCUO launched and console players are still getting screwed over when it comes to deals and sales. Stuff like Double/Triple Station Cash are non existent on the PS3 side cause it uses the PSN and the PS/SEN Wallet to fund all purchases. PS3 subs are also gimped cause of this since their Loyalty Points and the limited selection of items you can purchase with them are a poor substitute for the monthly Station Cash reward the PC subs have. Sales have also been problematic in the past, but I will admit they have gotten better at offering them on the PS3 side. These issues are annoying in DCUO's segregated servers, in a cross platform environment they would rear their ugly head front and center every time the Station side has a promo going on.