Just a few thoughts from an old 3rd kyu:
As Boon pointed out, tai-sabaki is not only footwork, but body movement. Your body is the centre of your universe. Whil you move your body, you move the (your) universe and an unharmonized attacker has to follow (unless you run away too fast
). So in this case tai-sabaki is all aikido - or as Stefan said, kind of dancing - as an exercise you dance alone, in aikido practice you dance with a partner, and - if and when you got it - you can dance with someone who does not intend to does, even doesn't realize dancing with you, while he wanted to harm you.
From my own experience - tai-sabaki as footwork AND body movement is very essential. I often see beginners, who seem to be nailed on the mat - feet fixed with cement or even concrete - no steps - no turn possible. I cannot believe, that I ever moved like this
. So obviously that is the first lesson from tai-sabaki: Move your feet freely!
But when someone starts to move his feet, his body is still doing strange movements - out of balance - (badly directed) power vs. power an so on. So after a few years I believe I managed this quite well - still some odds and missing connections to the partner's ki, but not too bad.
Recently I started doing some advanced kumi-tachi with higher speed. And it seems as if there was glue on the mat. While concentrating on bokken and body, my feet only move slowly, they are not free. And then my centre does not move as it should, too much bound to Newton's lex inertiae, as well going back after proceeding, changing direction, turning, etc.
So yes, while there are some needs to know how to do the technique, the key is body movement (tai-sabaki).
But my bigger problem in aikido seems to be even more important, is perceiving uke's ki, power, strength and direction, how to react to it or even lead it. Starting to achieve this is the key to do good technique on non-cooperative (uncommon or unused behaving) uke. Mastering it would probably even been to get beyond the need of doing technique, you just move appropriately.
And just one other point to mention:
whatever truth I found, was true - for that moment. Some small parts persisted a while. But very often the truth I saw a few months or a year later, was totally different from that before. Very often I do not care, as I do not recall my 'old truth', but sometimes I am reminded to it by people stating something, I said or would have said similarily before. And now I think totally different. And my teacher's comments seem nearly to be the opposite of what they were a year before. It can be irritating, but at last it is always the truth for one moment - one state of skill, ability, knowledge, and insight.