I don't think anything is wrong with ki. I strongly suggest that people read The Spirit of Aikido by Kisshomaru Ueshiba.
In that book, second doshu has a very rational and lucid discussion on ki and its various meanings. In the end, what I took from it is that ki can be thought of as focus or intention; a way of uniting the mind and the body.
A great analogy would be the way a baseball player continues his swing even after hitting the ball. His intention is to swing through the ball. In terms of physics, the portion of his swing after the ball has connected does absolutely nothing to affect the ball. But, if the batter only swings until he hits the ball and then stops, his mind will have slowed the bat part way through the swing. The follow through is like ki -- it units the batter's mind and body so that the bat is swung powerfully as it hits the ball.
Similarly, when I was shooting competitively, I always held my sight picture until well after the bullet had left he barrel. Had I been thinking of checking the spotting scope immediately after pulling the trigger, my mind would get ahead of the gun and I'd start to move while the bullet was still in the barrel. Again, the follow through was like ki in that it kept me focused long enough for me not to mess up the shot.
These are physical phenomena that are affected by mental intention. Just like ki.
So in aikido, I seldom think just in terms of ki, but it helps as a mental model for ensuring proper body position, structure, and movement.