I really like the convenience of the infant seat. It gives you a place to hold the baby when you don't want to sling/front carry him (like in a restaurant) and as Bazanna said, it's nice to have the seat in the house where it's climate-controlled.
Some techs strictly prefer a rear-facing convertible seat because they mistrust the base part of the infant seat. I've queried a list of techs from across the country to find out if there have been any recent cases of an infant seat being thrown from the base in a crash and no one could come up with an example. I'm perfectly satisfied with their safety from that viewpoint, as long as the base is installed correctly and the infant seat is locked into the base each ride.
Infant seats also tend to fit newborns better, IMO. The Marathon's lowest harness slots are quite high and I doubt a newborn would fit properly in the seat (for a rear-facing child, the harness slots used must be at or below the baby's shoulders).
As for the LATCH issue, all car seats (infant seat bases included) manufactured beginning in Sept. 2002 must come equipped with LATCH connectors. Since your vehicles don't have LATCH, just tuck the connectors out of the way and use the regular vehicle seat belt.