Nerd, I think you mean a handful of people using a definition not supported by major dictionaries such as the OED (even more limited than American Memory, it specifies "one side of paper only") and Webster's (a sheet printed on "one or both sides...").
I see no need, on the part of a student of song, for a term more limiting than- a sheet printed on one or both... containing a poem, a song or songs. Of course modifiers, such as 'blackletter,' are useful in defining a printing period or other feature that groups these ephemeral printings.
Bronson, in "The Ballad as Song," apparently makes no more of them than as a carrier of a poem or song.
The Bodleian Collection, of course, is a provincial collection, restricted to ballads in English; as the introduction states: "Broadside ballads were popular songs, sold for a penny or half-penny in the streets of towns and villages around Britain...." Of course these sheets in part were distributed around the English-speaking world, so their influence extends beyond "Britain."

I doubt that your definition has much currency in the United States. You also use the limiting term 'narrative,' which would eliminate part of the Bodleian Collection, Murray Collection, etc.