What this means is that unionization can be forced on 49 workers by the vote of 51. This type of system disenfranchises and forces fees of those who may not regard the union as a benefit in the workplace.
There is an odd sort of thinking that has been going on in libertarian circles about just how free the right of entry or exit should be. In this case (a nearly evenly divided workforce), one side or the other is not going to get the outcome that they desire. Why do we privilege the "don't want a union" side over the "want a union" side?
This seems to be the same issue with democracy. There are lots of decisions made in a democratic state that I do not agree with. But at some point we need to decide how to organize groups. Giving a small group veto power over change simply removes adaptability from the society as whole.
Now in terms of the post, itself, minority unions might be fine. What one has to deal with are the asymmetries of power in the situation. A recent discussion about these issues asked if a boss could make sexual relations part of the employment situation, on the premise that the employee always had the right to quit their job. In a world without leases and with easy employment this might be true, but it clearly does not define the actual world we live in. In the same sense, minority unions would be very interesting ideas in a world where the employer could not just ignore the striking minority union and hire more non-union workers.