the reason why petrov isn't up to the job yet is the same reason why williams decided to keep barichello rather than hulkenberg, one year's experience in F1 does not make you sufficently knowlageable to develope the car, especially now that testing is so restricted. renault can not afford to go down any routes in development that does not work and similarly they can't afford to reject a possibility just becasue at first it doesn't seem to be beneficial. the trick is seperating the one from the other and certain drivers have the ability to do this more reliably than others, williams always rated heidfeld's feedback on developments and admitted it's one area they struggled with after he left. senna might possibly be faster than heidfeld in an equal car but it's very likely that heidfeld would ensure his car was substantially faster than senna's by the fifth race if they both developed a car seperately.
also with the points system the way it is and changes in the regs, the ability to bring a car home will count for a lot, petrov had this last season in some races but senna made a ofof mistakes through pushing past what the car was capable of. it maybe that in a more capable car he'd be a better finisher but when coupled with petrov's habit last year of hindering his race prospects through crashes im qualifying / practice it's too much of a gamble to have both of them in a team at once, thats why teams tend to like one steady driver alongside a more "advetureous" driver.
ideally from renault's poit of view, heidfeld will develope the car and bring it home in races whilst petrov will build on the flair he showed last year and be challenging at the front.
of course the worst case scenario from testing is if senna and heidfeld are both faster than petrov !
[small fuel tank?]
