The benefits of collective bargaining stem from the individual's freedom to withhold their labor. Or are you arguing it's somehow unjust for people to refuse to work with non-union coworkers?
Compelling an individual to support a group runs against the concept of individual freedom. I support the right to form a union and the right to not join one. I do not support the right to require an individual to join.
I support the individual. Pretty simple.
Does this mean you don't support the individual's right to refuse to work with non-union coworkers? It's not as simple as you think.
A group should not be able to exclude the individual if he/she doesn't want to be part of the union. Period.