You're right about atmospheric flight.
Like I said, the show wasn't very good, it was made in the early seventies. The main purpose for the Eagles (that's what they were called) was as the workhorse for moonbase Alpha. The center module could be changed out for various missions as the craft had a flying crane configuration. Plus with the entire craft having a modular construction, one could be placed in Earth orbit with four or five shuttle launches and assembled at the ISS. It's just a very cool looking ship. :)