I've always been a big fan of the Doobies, especially of their vocals, so I was intrigued about how they were layered and asked to have them soloed. Much to my surprise, the vocals weren't layered, doubled or even treated except for printed reverb - just pure 3 part harmony (two parts above the lead vocal). What I especially loved was how imperfect they were, with the highest voice straining to hit the notes, coughing and throat clearing in between phrases, and even a little talking on the track.
How refreshing! Nowadays we make everything perfect because we can, more often then not to the detriment of the music. Way back when the technology wasn't nearly as advanced as the lowliest DAW in the typical bedroom, perfection didn't matter, but feel and groove did. There were lots of mistakes left on tape, both musical and technical, but they added to the character of the music. Today we get rid of the character as a matter of good production policy, yet the songs from that era endure and our modern more perfect ones don't.
The lesson here is that no matter what, a hit is still "in the grooves" (to use an old term relating to vinyl records). A hit song with imperfections is still a hit, while no matter how perfect a mediocre track might be, a hit it's not.