My point is that there is a very big difference between aiki and aikido. Just because you see aiki in something doesn't mean there is aikido there. To say that MMA fighters (most of whom have never trained aikido) are doing aikido makes about as much sense as to say that every musician who improvises is playing jazz. Sure, improvisation is a foundational, unifiying concept of jazz, but that doesn't mean jazz has a monopoly.
It's just a metaphor for the similarities; furthermore, what you call aiki, others might not call aiki. So the question to my mind isn't whether we can pinpoint a truly objective definition (good luck), but whether we can understand the semantics intended...which is always a process, particularly the closer we try to look at something. My current definition of Aikido (i.e. Ueshiba Aikido) is the study deriving from Ueshiba Morihei's lineage of teaching. It would leave a lot of room for interpretation if we were going to base it on the representative behaviors available, which, depending on your school of thought regarding "aiki" itself, may or may not actually include aiki.
That all said, I love how often Aikido is derided for its "grab my wrist" mode of engagement (because apparently no one ever grabs your wrist in a "real" fight), but I've seen plenty of wrist/forearm engagements in the handful of MMA matches I've seen...of course I have yet to see (or to recognize, at least) a viable connection to center through that grab/suppression. "Aikido is all over the place" because we all share the same human form and pretty much every martial art will overlap based on that.
My mid-stream-tossed wooden nickle...