I dabble in creating mashups from time to time [1] and I see that first hand. One of my first was built around Let's Groove by Earth Wind and Fire. As a funk act, they are as tight as a group of musicians can ever hope to get, and still, the tempo would subtly speed up and down from bar to bar.

It makes things much more challenging to sync up! You can't just speed up the slower song (or vice versa), because even if the averages match, it will still be all over the place from bar to bar. I now have much better workflows to deal with this, but for that first one, I spent a looong time manually adjusting things to get them to fit.

Working with tracks that were recorded to a click (or digitally gridified, as per the article), are much easier to work with. Match the tempos and everything automatically works. Definitely helps creating my mashups...

But I do agree with the post, in that it loses some indescribable human aspect. I love the push and pull of musicians trying to follow each other rather than concentrating on the click. Or even just pushing and pulling against oneself with things like rubato in classical music, sometimes accelerating, and sometimes slowing, at times almost to a dead stop, so as to build a sense of antecip.... pation.

[1] Link if anyone is interested in checking it out: https://www.youtube.com/watch?v=fP74tMiXBU8&list=PLsj2XVeWwU...