I would think a company would prefer to deal with a fleet owner, however, like it was said in the first reply, if that fleet owner gets erked off he could pull the whole fleet and leave the company in a tad of a pickle.
But, as a fleet owner, one might consider placing say 10 trucks over with company ABC and the other 10 trucks over with XYZ, thereby having a little bit of muscle and pull with two different companies.
Bottom line, for the company I see advantages and downsides on dealing with the fleet owner. But if you have 20 bad drivers you still have 20 problems to take care of as a company no matter if its 20 calls to one owner or 20 calls to several owners.
Point is - the company will want to make sure the drivers in the fleet meet their standards no matter what - because the key to a successful fleet or to the company is the person in the truck.