There is more to this than advertising the new ride and there is way more to this than a bored PR department wondering what to do with their time.
Was talking about this with people on Sunday whilst at Chessington, it really seems that the logic with this is pushing each ride to have a brand identity. They are trying to make the general public aware of the differences between the rides, not just their physical differences, but their thematic ones. Whether or not it will work, or is working, is beyond me. Nemesis is in the lead by a long way, and it's likely the majority of votes are from enthusiasts. Especially since you can vote more than once.
But the effort behind it is promising. It means they care about and think that the likes of Oblivion and Nemesis, in particular, are strong enough brands within themselves to get the public to recognise. It also means they may have a restored interest and confidence in doing that kind of thing again and again (as we are seeing with Thirteen, Raptor, Krake and now Swarm). Rides like Rita and the other examples around the parks where the names and themes were obscure and didn't fit, were attempts to be memorable by being quirky. It was a trend that at the time was becoming popular in all areas of popular culture, when random, unbranded, nonsense, generic imagery was cool. It was and still is particularly noticeable within illustration and the ultimate example is on t-shirts. Go into somewhere like Primark or even Top Man and look at the irrelevant crap on the t-shirts. This is cheap, simple and applies to a broad audience. But I think this age is starting to die down now, and people are beginning to take more value in effort and meaning again.
I think the reappearance of a Nemesis t-shirt this year is part of this revaluation of how the park can revitalise their existing rides, not just bring guests back with the new. I bet we see an improvement in merchandising for them all next year.
EDIT: As for nothing new this year, that's just because Dreamworks are dickheads and let them down after they'd been working on this years addition for months. At the time when I first heard about the Shrek stuff, I was dubious, but now I actually think it could have been awesome. Shame, but in the long run I hope it pays off. The next few years should be good.