Indeed but Senna is scoring points and stays consistant something which teams find important if they will be fighting for the constructurs championship. (not to mention some sponsors pay if a driver finishes within the points)
We know that Senna maybe doesn't got the Qualify pace but I find this situation the same as Hamilton and Button, they both drive the same car, Hamilton seems to be much better in Qualify while Button turns the tables during the race and is always picking the right strategy to turn the table and is quite consistant while we know Hamilton is unpredictable.
IMO Maldonado has more potential.
Still, Williams has a decent driver lineup. But their drivers lack F1 experience.