There's no doubt that Rossi would get tons of publicity for Ferrari and Marlboro (yes, a tobacco sponsor!). But to satisfy the tifosi, the legions of Ferrari fans around the world, a driver needs something more than commercial appeal: he's got to, well, win races. And that's a bit of a shortcoming on Rossi's résumé at the moment: he's never actually won a race in a 4-wheeled vehicle. And, AFAIK, he's never even competed in an open-wheeled car.
Rossi and his well-oiled PR machine, campaigning at full volume, seem to be trying to get Vale signed by Ferrari for 2007, after Schumacher retires. Without a single car race on his record. No F3. No F3000. No GP2. Not even an apprenticeship with another F1 team. This is so utterly absurd that it defies comprehension.
A Formula 1 car corners at twice the speed as a MotoGP bike and brakes from high speed in one-fourth the distance. While I think that Rossi is capable of mastering the much different skills that F1 requires, it's going to take a lot more time and effort to reach that level than he seems to think will be necessary. A few weeks of off-season practice this year and next before the '07 season begins won't do it. The drivers he would be competing against are no less gifted than Vale, but they've spent the past 10 years or more mastering the skills of F1 while he was mastering the skills of MotoGP. That's a helluva lot of ground to spot the competition and still hope to succeed.
