The union’s too weak to help me, what is the point in joining?
The union's strength depends to a large extent on how many people join it. It is a fact of life that the more employees join the union the more the employer is prepared to listen to it.
The more members there are in a place of work the more local management realise that they have to be reasonable in how they deal with members.
You can make the union stronger just by joining it. You can make the union even stronger by joining and taking some part in the union's activities, as much or as little as you choose.