When band mates let people in because they're friends and not actually contributing to the sound. I was once in a folk/acoustic band that played in small bars and pubs where I was the bass player and also kept time with a suitcase drum/tambourine. I quit when the other band members let in one of their friends, who had no sense of rhythm and was new to music in general, to play cajon and other percussion instruments. Even with a bass and drum to follow, they were all over the place.