Yeah, but that starts to walk right up to the 'right to work' line, give me the benefits of a union shop without the responsibilities like voting, dues, or solidarity of action until later. If a union is in place you already have benefits of it most likely by having a guaranteed raise schedule or higher starting wages than you otherwise would. If it's just being established you'll have a vote, and if it gets established against your vote then that's just the way it is and you either join or leave.
The dues are integral towards the operation by funding things like a strike fund, hiring negotiators, or any other operational costs.
Think the other side of it, you get hired and floated a 'loan' until some later point. In the interim a strike is called. Are you going to walk out and expect pay from the union from a fund you haven't paid into?