It's said to be bad for the worker because its bad for the unions. Anything bad for the union is said to be bad for the worker.
If people are allowed to choose not to join the union and not pay dues a lot of people will opt to not join the union and not pay union dues. Doing so will reduce the amount of money the union receives and the unions will claim that that minimizes the necessary funding to retain attorneys and pay for collective bargaining and arbitration.
If you work for a company that requires you to pay union dues regardless of your union membership why not join the union? If you do not join the union you are not entitled to union representation in the event that your job is threatened. This is often something union afterwards say to people who don't want to join the union and more often than not it is enough to get them to join the Union bOn the flip side you don't have to go on strike if the union calls a labor stoppage.
Personally I'm all for right to work. A person should have a choice but I'm also not a fan of labor unions so my opinion is bias.