Practice sessions involve the same time commitment for everyone in the band, but the preparations for that practice are vastly different. Everybody brings a different understanding.
Some people in the band may have played the same three or four chords their whole life, and that covers most of what the band does. For them, a lot of practice can be eliminated by using the Nashville Number system. Whoever knows the song communicates it to the rest of the band and very little rehearsal would be necessary.
Some people in the band might listen to the originals of cover tunes as preparation to practice. You can learn a lot by listening, but it often leads to unnecessary discussion, when different members hear a song a different way, or want to emphasize one aspect over the other. The band that wants to develop their own interpretation needs a leader who is also an arranger or has the financial resources to put the desired arrangement into a form acceptable to the musicians. Otherwise this is what most of the practice time will be devoted to. The net result might be a good level of precision, but one that could be just as easily achieved in less time with arrangements.
In the case of blues, r&b, rock or other dance music, some of the best players need little or no practice. They really only need to rehearse the beginnings, endings, and stop times or other aural tricks in the song.
It might surprise someone to hear that a fun practice is a bad practice. Doing it for fun is the reason most musicians play, especially since live music gigs have gotten scarce. I submit that practice makes worse.
One typical situation is playing a song all the way through. Most of the time, a singer wants to do this. Hey, who wouldn't? You're singing, and you've got a live band behind you. It may not be as good as karaoke, but that band is also your built-in audience.
Typically, that leads to playing the song again, usually because the guitar player has been adjusting their volume during the first time through, and one more time, they might think they can get it right.
At this point, there might be a group effort to get the sound right, maybe by playing the song again, or moving onto another song. What might enter in, is the sound reinforcement aspect. Amplified music has made things so complicated that most hobby players will waste more time and money on gear than will ever be compensated by improved performance.
Alongside the issue of what amounts to wasting time on a "gig scrimmage" is, the players may be friends and actually have no objection to coming up with a new take on an old chestnut. Music does bring people together; there's no arguing that. Like anything else though, compromise is inevitable. Meshing various skill levels works only for the people on the low end of skills. Like many musicians, I've always treasured being the worst player in the group, it actually means you're going somewhere, getting to play with a higher level of musician than oneself. By the same token, being one of the better players in the group often aggravates bad behavior, either by those who can't cut it, or by the good player themselves who becomes morose or passive aggressive.
Hopefully I've laid out a few simple considerations to show that "practice makes worse." It sounds counter intuitive because people think when they invest time, things get better. Let's say you mow your lawn and all you have to show for it is a bunch of cut grass. Well, you spent the time on it so you think it looks better, right? Just like a haircut, the answer is no. It won't look right for a couple of days. One of the reasons smart bands only rehearse when a gig is a couple days out.
Another consideration I haven't mentioned has evolved in the last 20 years or so. It is the ubiquity of song lyrics easily printable from the internet. This has led to an explosion of people who see these lyric sheets as something to read during a performance, rather than something to memorize. It's easy to understand why superstars do this, they are paid well to not forget lyrics, especially their hits. During the first few rehearsals of a song, any kind of chart (as mentioned above) can ease the need for practice time. When the lyric sheet doesn't go away after a certain amount of practices and/or gigs, the rest of the musicians can sense the singer's lack of commitment. Everyone might react differently. I'm a bass player, and I'm no saint, so I start listening to just myself, and perhaps the drummer if I like what they are doing. The singer's gonna keep on reading their sheet and the guitar or keyboard player's gonna keep on adjusting their volume. That's when it's time to stop practicing and let the gigs be the practice.
Here's a couple of good links:
That's a quote from Jill Jaxx at her site http://learntosingbetterfast.com/
She also has a video on youtube.
This is a good link describing the need and tactics for memorization: http://www.singingwood.com/NewFiles/CarnegiePractice.html
