Do you remember the film "Jaws"? The shark was never seen until three quarters into the movie. I think the fear of the unknown makes the ride more exciting- think back to the first time you went on the ride, unless of course you ingnored the spoiler warnings & knew what was going to happen- the fun of not knowing was great. Yes it would be really neat to get a good look at the Yeti but I think it would ruin the fun.
There's a couple of problems, though, with the "Jaws" comparison. First of all, with a movie you have around 2 hours to build the story up, build suspense, get involved in the story, etc. On a 3 to 4 minute ride, you don't have that luxury of time. You need to satisfy the audience more quickly.
Second, in "Jaws" you got to see much more of the shark as the movie progressed. By the time we get to the second half of the movie, we've already seen the shark several times, and by the last third or so of the movie, we've gotten several good long looks at the shark. The movie would have left audiences feeling frustrated if we never got to see the shark throughout the entire movie (except maybe a cartoon shadow) and then at the very end we just got a 2 second glimpse. I don't think people would have come away saying, "Wow, that was really suspenseful, and now I want to see the movie again and again just see what I missed." I think people would have come away thinking it was just a low budget flick. By only giving us a brief look at the Yeti at the end of the ride, that's essentially the effect Everest has. It makes the ride come across as cheap.
Now let's take another movie comparison: "Jurassic Park". In that film, they don't just show brief shots of the dinosaurs. We see them throughout the film and most of them are scary. Additionally, when "Jurassic Park" first came out, the digital technology they used to create the dinosaurs was definitely considered very impressive, and something the filmmakers wanted to showcase to a great extent. If the Yeti on Everest is the AA equivalent of that (i.e. If the Yeti is truly as advanced as Disney and fans claim it is) then I think they would want to show it off a little more.
It's probably also worth noting that if you've seen any documentaries on the making of "Jaws", then you probably have heard about all the constant complications they had with the mechanical shark in that movie. It just wasn't nearly as sophisticated a prop as it was originally intended to be, and so Spielberg had to get pretty creative at times to work around that problem. I've often wondered if the same thing happened with the Yeti AA. Maybe it was supposed to be more sophisticated and they ended up having problems with it, and consequently they placed it at the end of a fast moving portion of the ride as a "workaround". Then they disguised the whole things as "this is great storytelling! Look at how we build suspense and make you want to keep coming back!"
