[quote=Ken Watts][quote="Жэнтос":325hbiz1]And then "be not"="ben't" is also possible, though also grammatically incorrect, isn't it?[/quote]Although I have never seen "ben't" until now, back in the 1600s it was used:
[quote]During the Restoration period--after 1660--we begin to find the first printed evidence of several negative contractions . . . [s]everal of these contractions look unfamiliar now: [i][color=red]ben't[/color], an't, en't, han't[/i]--these have either been replaced by other, newer contractions or have simply gone out of use.[/quote]The Merriam-Webster New Book of Word Histories (1991) [url="http://tiny.cc/sj7tc"]http://tiny.cc/sj7tc[/url] (also includes the history of "ain't" if you are interested)[/quote:325hbiz1]

I think that "ben't" probably went out of usage due to the fact that most English speakers use the phrase "do not be" rather than "be not"

Example: "Be not sad" vs. "Do not be sad"

Just a guess though.