As with any governance, the best governance in business is an intelligent and benevolent dictatorship. The inevitable occurrence of ruthless or dim dictators means that employees will come together to protect their interests. When they do so come together, business change becomes much more difficult, which is more often bad than good in a fast moving economy. The argument then is that if you want people to not unionize for the sake of agility in business, you have to allow them alternatives to living under a dictatorship which may be ruthless or dim.
“At Will” employees are often not simply free to go. These employees are often most familiar with matters that are of intellectual property to the company in question and cannot so easily use their transferred knowledge without consequences. Moreover, companies often force employees to sign covenants not to compete, which entail that people do not join competitors or otherwise enter their marketplace if they were to leave.
Intellectual property is not going away, and shouldn’t, but if you think unions hurt business, you have to allow employee mobility as the alternative. This can be done by not forcing people into covenants not to compete.
If you like unions in themselves, I don’t think you understand economics. If what you want is a collective power, then partner and hire employees yourself to develop alternative or complimentary products and services.
Workers should be allowed to do this much and a leadership’s best defense is being intelligent and benevolent.