You are correct, about the openings having functionality. The only parts of kata that don't have a real physical function are the hand SYMBOLS that some arts make before and after a form is performed. These serve a spiritual purpose and moral purpose. There are many different definitions for each hand symbol, depending on the martial art and religious beliefs, and that can lead to an endless debate, going back into the fuzzy history of martial arts (because it will lead there and the very origins are lost to history).
The 'preparatory' stances as some arts call them, and the arm movements, and hand postures leading into the kata are very functional and serve a self defense purpose.
EDIT -
Just so you understand what I mean by hand symbols that don't have martial application. Here's a link to a mudra from Aikido:
http://griffithaikido.com/brisbaneyoga.html
This hand movement is very much a martial application:
http://www.sonieshine.com/Sonie_mudra_web.jpg
EDIT- Shiro Kuma
You're right about Ueshiba Morihei, being of the Omoto Kyo sect of Shinto. While there were a few body rituals that had the same goal as mudra from the Buddhist paths they were by and large different, or just non-existent in Aikido.
However, since the mudra are a religious ideal, I have seen them employed in several schools and during many of the various demonstrations I've been to. Which isn't really much of a surprise as the art has grown, and is bound to cross into people still following the more esoteric Buddhist sect (mikkyo), or are from another Koryu art, before teaching Aikido.
The particular picture I posted was the first real mudra that I saw when I looked them up for something to reference. Everything else was Naruto garbage, and I couldn't post something like that where people who are so impressionable can see and use as fact. (Even though, some of the mudra are based in reality.)
I have always found mudra to be interesting, and pointless at the same time. Really, its all based off of religion. Although, as I'm writing this, I get to thinking that mudra is prayer, so depending on who you pray to, a quick hand gesture may be better than reciting a long paragraph when faced with danger... hmm
One funny thing about it is that the most well known 'old school' (as a real Koryu) Tenshin Shoden Katori Shinto Ryu still practices the use of mudra, even to the point of displaying mudra in the middle of some of its drills and kata.
... Something funny and circumstantial with mudra again is the national part of my system Shuri Ryu. The head instructors are mostly strong Christians, and yet they teach and perform the mudra before every kata, and demonstration. They know the meaning of each hand symbol too. And yet, they pray in Christianity before every class and event.