In a conventional at-will master/servant relationship (what most assholes call "employment"), the employer holds all of the cards. The Courts generally will uphold the employer's assesment of your qualifications and job performance and will uphold the employer's right to fire you whenever it wants for whatever reason it wants. But...

When you are a HO and you have multiple employers, thangs are different.

For example, let say asshole employer "A" decides that you are not qualified to turn oxygen into carbon dioxide and fires you. Well, since you have more than one employer and are doing the same work for several different companies concurrently, the Courts may find that asshole employer "A" is the one with the problem.

Also, when you have several employers and one decides that it can decide if you are or are not qualified, you have legal grounds for suing Asshole Employer "A" for interfering with your contractual relationships with your other employers. By asshole employer "A" claiming that you are not qualified, asshole employer "A" is interfering with your other contracts and your ability to engage in the business of contracting with others.

In short, the more employers you have, the more stable your life will be and the better chance you will have of destroying the life of one of your supervisors if he/she looks at you the wrong way.