It seems to be archaic, used only in old quotes like nursery rhyme

Hoowever: in song or poem this "a-" gives the writer the extra syllable needed for the meter scheme. (Akin to a poet using "doth go" instead of "goes".)

But is the a- used outside of song/poem, where meter doesn't control? "Mr. A-, I ask you, 'Are you now or have you ever been a member of an ordinary dialect?'"