No offense to Dario, but he should have stayed in the IRL. I think he could become a decent Cup driver with the right team. Ganassi isn't that team. I never saw why Dario had the interest in NASCAR anyway.
He wanted a new challange from what he said. But you are right, Ganassi is not the team to try your shot at nascar with. They are pumping all they have, on the nascar side, into keeping Montoya in the top 20. Whatever is left over might go to Sorenson or whoever the poor guy is that will be his replacement next year.
Dario, and other open wheelers, came to NASCAR for many reasons, but probably the kicker was the somewhat success of JPM. The open wheel series have been losing ground over the years to NASCAR and so naturally, that is a big attraction to any driver. But as has been said, success also has to come from the team and Ganassi just isn't a top notch team.