Want to hear a really sloppy record? It's a good song, but the recording's a mess. The drums consistently drag the rhythm; the bass player isn't quite sure how his part is supposed to go. If you listen carefully to the end of the second verse (around the 48-second mark in this video), the whole band gets lost for a moment and ends up adding an extra beat by accident.
It is, of course, The Beatles' "Rain," as great a rock recording as anyone's ever made. And it's full of mistakes, accidents and inconsistencies that would be utterly unacceptable by the pop-music standards of 2009.
Now imagine what would happen if some band of 25-year-olds with radio aspirations wrote and recorded "Rain" today. That take would probably be thrown out, or at least digitally edited to fix the screw-up; even if they played it right, the drum track would get imported into ProTools and snapped back into strict rhythm any time it drifts behind the beat. The lead singer's wobbly notes, and the not-quite-in-tune bass guitar, would get fixed with AutoTune. The all-over-the-place guitar dynamics would be tightened up with a compressor-limiter. It'd still be a fine song, but the recording would be impossibly boring — as frictionless and dull as the recordings even the best mainstream rock bands often end up making now.
Voices, guitars and drums are really expressive instruments for the same reason that they're really inexact instruments: Tou can't coax the same note or beat out of them exactly the same way twice, even if you try. They're never perfectly in tune, and any number of factors can throw their sound a little bit off. Add that to the fact that, if you're working with analog tape (as almost all pop musicians did before the mid-'80s), you're basically stuck with the performance you've got, and you end up with recordings that mercilessly document endless errors, small and large.
The large ones sometimes used to make it out into the world; occasionally, they even turned up in hits. James Brown's "Sex Machine" has a wreck of a keyboard solo; the Mamas and the Papas' "I Saw Her Again" includes Denny Doherty flubbing the beginning of a chorus. That doesn't happen anymore, and hasn't since massively multitrack recording became standard operating procedure for pop: Losing an error doesn't mean abandoning a group's entire performance.
Another kind of inconsistency has been methodically crushed over the last two decades, literally and figuratively, in what audio engineers call the "loudness wars": the competition for new recordings to be as loud as possible. If a piece of music can be compressed to a very narrow dynamic range — a minimal distance between its quietest parts and its loudest parts — then that means the whole thing can be really, really loud. It sounds bold and forceful when you put on a CD or play an MP3 file; it's clearer and less likely to flicker out when it's played on the radio. If you listen to a super-compressed, very loud recording next to an uncompressed version of the same thing with a wider dynamic range, the louder one is going to seem much more immediate and consistent. It's also going to be harder to listen to at length, because the natural dynamics of rock groups — not just the difference between quiet parts and loud parts of a song, but also the accidental fluctuation from one moment to the next — suffocate when they're squashed. Here's a video that demonstrates the damage the loudness wars have inflicted:
And now, the smallest errors are vanishing, too. The gift that modern digital technology has given pop music is the ability to fix every nagging inconsistency in a recording, note by note and beat by beat. If you hear a contemporary mainstream rock record, you're almost certainly hearing something that has been digitally nipped and tucked and buffed until it shines.
The little inconsistencies in musicians' performances aren't just glitches, though: They're exactly what we respond to as listeners — the part that feels like "style," or even like "rock." The exciting part of guitar-bass-drum-voice music is the alchemy of specific musicians playing with each other, and the way those musicians' idiosyncratic senses of timing and articulation and emphasis relate to each other. That's where the rhythmic force of rock 'n' roll comes from; that's also why a great band can replace one of its members with someone who's technically a more skillful musician, only to discover that their instrumental chemistry isn't there anymore.
Fix enough little mistakes and inconsistencies in a rock recording — snap its rhythms and pitches to a grid — and you've effectively replaced all of a band's members with technically more skillful musicians. That can be very useful for some kinds of pop, especially kinds in which a well-executed composition is more important than an idiosyncratic performance. On the other hand, it's dangerous or even fatal to recordings in traditions like rock — the kinds of music that thrive on friction. The high-tech ideal of popular music means no botched rhythms, no sour notes, no shaky dynamics, but also no "Sex Machine," no "Louie Louie," no "Rain." It's not always worth the trade-off.