Saggy, let me put it to you like this: I know a pastor that once thought his church building was too small. He had an idea, though: Start sending his church members one by one over to the church building that he really liked, and have them become members. When enough of them were over there for a majority, they would then vote him in as pastor, and they'd have their new church building.
When we all started laughing at him, that's when he said, "Oh, yeah, I was just joking."
Voting on membership gives members who might be "in the know" the chance to bring to light anything that might actually prohibit someone from membership in the church--such as, someone who professes to have been baptized and is being put forward as a member based on their statement of faith and baptism, but you know for a fact (because you've lived near or even with that person all your life) that their statement is false.