While some companies do use freelancers, they tend to use a few rather than lots, so the faces are familiar and the pilots tend to have reasonably regular work with that operator. I get the impression from my freelancing friends that the companies do this as a way of providing a fairly stable work force but without the risks associated with having to lay off when things turn difficult or if the aircraft is at risk of going away.
I don't think that freelancing is a preferred method of staffing an aircraft. Recently I was put in a position where I had to look for another job. On finding out which companies used freelancers on my type, I actually found that they would prefer to hire me full time. So while the money is less, the security for me is better. I found that a big surprise and very interesting.