(My opinion)
As an owner operator, the goal is to build your business. If a specific company is not providing you with a means to do that, then by all means...it makes sense to step outside the box to keep yourselves busy. Does this mean leasing onto 2, 3, 4, carriers? Leasing onto more than 1 company is similar to a carrier going out and getting a bunch of customers. Instead of waiting for 1 customer to call, they have 10 customers that could call. In essence, when you, the owner operator are leasing onto more than 1 company, you do with the intentions that you will have more than 1 person potentially looking for loads for you.
The other point of view is that of the company and the people who work at or with the company and no other company. Speaking from experience as a dispatch agent who currently works with a smaller carrier who allows owner operators to lease onto more than one company, I would say that it can be very frustrating focusing time and energy into someone who is not 100% committed to you and the company you both are working with. Thus, those people who do lease onto 2, 3, 4, companies may end up taking a backseat to the committed owner operators that the dispatcher knows are committed to their company. If I have 3 owner operators sitting in a specific area ready to go and 1 of the 3 is leased onto me solely....who do you think will get the first call and the first right of refusal?