Disney's attempt was a pretty flagrant abuse of the law (definitely the spirit of the law, if perhaps not the letter), as was Edison's, and occurred precisely *because* it's actually difficult to find highly trained workers in the US that will work for peanuts. So, I'm not so sure that's a great example. The solution to H1B abuse isn't to unionize, but to fix the H1B laws.
The ability to move from job to job has enhanced my earning power and career opportunities, personally speaking. Each move to a new company was also accompanied by a better wage, better company and working conditions, better benefits, and more responsibilities. Yes, I've been laid off as well, and that's never fun, but in general, I think not being tied to a union has been a positive thing for me.
Reasonable frequent migration between companies is also a good source of idea and methodology cross-pollination. I've seen people that stick to a single company for most of their career, and in my opinion, their thinking often tends to be the most rigid - less able to adapt to new and better ideas. Granted, new isn't always better, but it's almost inevitable that *eventually* a new and better way will be found for doing almost *everything*.