Indeed. While he does have some skill, Speed is far from being polished and experienced enough to really make much of an impact on F1. Especially with the fiasco that happened at the 2005 U.S. Grand Prix, F1 needs as much good publicity in the States as it can get. That's why Speed was brought up and given an F1 ride.
Most good drivers will spend a year or two being a team's test driver (like De La Rossa or Maasa), where they develop their skills. If they don't do that, then they're good enough to be given a ride and they make an immediate impact (like Alonso and Raikkonen did) as soon as they debut in their first season or two. Speed never spent any time as a test driver, and he certainly hasn't made much of a significant impact (other than when he causes the occasional accident due to his inexperience) so far this season.
The last time there was an American driver in F1 was about 15 years ago with Michael Andretti (who drove for McLaren). He never really fared that well in F1, which is why after a few years there he left and went back State-side and drove IndyCars.