I don't believe that Maybelle played Autoharp with the original Carter Family. Sarah was the Autoharp player. She only played guitar when the Autoharp didn't have the required chords and she played it lying flat, not against her chest.
Maybelle took up the Autoharp in later groups and played it against her chest.
I like being able to move into the mic for solos and away for back-up, agood reason for playing against the chest. This is also one of many reasons that I don't like using pick-ups on Autoharp, guitar, banjo, mandolin...

By the way, how many players use straps on their Autoharps? I've never found one necessary.