It's not practical to opt out of various benefits that come from the union's activities. For instance, your employer's not going to be able to cut your pay/benefits much below that of union workers, or try to give you an abusive schedule/workload, since if they try you'll (I assume) just join the union at the first opportunity.
Further, the union may negotiate things like compensation and benefits packages for all workers because not doing so would terminally weaken them by letting management entice workers to leave the union, until it eventually dies—and then management can enjoy the deeply unequal bargaining position of giant-organization versus uncoordinated individuals, until workers organize again—but this time management can kill it more easily and inexpensively before it even gets started, through he same process, but with the bonus that this time they can also be much more coercive and punitive in their approach toward certain especially troublesome individuals since they have less-effective oversight.
[EDIT] bah, I effed up my reasoning on that second part by starting with one thing then going another direction and not fixing the first bit. There are two separate problems: 1) if non-union folks don't have to pay dues, the employer can kill the union by enticing people away from it, and 2) if they can hire non-union at below-union compensation (say, if you have to pick up-front whether you're union and can't sign on later) then they can simply only hire people who don't join the union (and preferentially retain non-union folks) until it dies.
Further, the union may negotiate things like compensation and benefits packages for all workers because not doing so would terminally weaken them by letting management entice workers to leave the union, until it eventually dies—and then management can enjoy the deeply unequal bargaining position of giant-organization versus uncoordinated individuals, until workers organize again—but this time management can kill it more easily and inexpensively before it even gets started, through he same process, but with the bonus that this time they can also be much more coercive and punitive in their approach toward certain especially troublesome individuals since they have less-effective oversight.
[EDIT] bah, I effed up my reasoning on that second part by starting with one thing then going another direction and not fixing the first bit. There are two separate problems: 1) if non-union folks don't have to pay dues, the employer can kill the union by enticing people away from it, and 2) if they can hire non-union at below-union compensation (say, if you have to pick up-front whether you're union and can't sign on later) then they can simply only hire people who don't join the union (and preferentially retain non-union folks) until it dies.