I imagine that the logic behind the reveal date is that more often than not, their more successful attractions (in Merlin's definition of successful) have come from when they've done a reveal in January time (the middle of closed season pretty much), meaning people get the instant hype and get ready to book, etc. When they've done reveals earlier, the public lose interest or the novelty wears off, losing that big hit.
Things like Smiler and Ghost Train did build a lot of hype and did get people booking (even if they opened late, etc). And the drip fed stuff prior kept interest going for enthusiasts too. It works. Certainly moreso than things like Swarm and Zufari which weren't as popular in Merlin’s eyes.
I fully expect a reveal of a name / story / promo art in January. We'll get the generic teasers, clues and whatever to 'tease the geeks' until then. That seems pointless when the geeks know the name and stuff thanks to the Internet, but there you go.