I disagree with the E190 being a "small jet". It is really close to the A318s being used by USAir. Maybe you are right about the JFK vs LGA thing, but not everyone lives in Manhattan, not everyone goes to work and then to the airport. I know alot of business individuals who live on Long Island and save time by flying out of JFK and leaving from home.
As far as the E190 flying shorter haul trips, true but I think it is a great idea that JB will place a more economical aircraft on the new cities until it is proven that they can fill an A320! I wouldn't expect JB to keep the E190 on BOS-AUS and JFK-BOS very long.
Lastly, the E190 is a brand new, "unproven" aircraft. I bet that JB wants to put time on these aircraft quickly and find the queertrons and querks ASAP.