documentclass{memoir}

usepackage{etoolbox}

newcounter{rownumbers}
newtoggle{showrownumber}%



pretotabular{%
    setcounter{rownumbers}{0}%
    toggletrue{showrownumber}%
}

newcommandifshowrownumber[1]{%
    iftoggle{showrownumber}{#1}{!}%
}
defnonum{%
    togglefalse{showrownumber}%
}
defrownumberdisplay{%
    ifshowrownumber{makebox[1.5em][r]{%
        stepcounter{rownumbers}arabic{rownumbers}.space%
    }}%
}
defrowfill{%
    ifshowrownumberdotfill%
    toggletrue{showrownumber}%
}



begin{document}

toggletrue{showrownumber}%
begin{tabularx}{.9textwidth}{>{rownumberdisplay}X<{rowfill} r r}%
toprule%
    nonum Name & A & B\
midrule%
    nonum emph{Section} \
    (1) Name & 100.0 & 3.5 \
    nonum -- and Name & 10.15 & 56.8 \
    nonum emph{Section} \
    (2) Name & 66.4 & 43.1 \
    (3) Name & 66.4 & 43.1 \
    (4) Name & 66.4 & 43.1 \
    (5) Name & 66.4 & 43.1 \
    (6) Name & 66.4 & 43.1 \
    (7) Name & 66.4 & 43.1 \
    (8) Name & 66.4 & 43.1 \
    (9) Name & 66.4 & 43.1 \
bottomrule%
end{tabularx}%

end{document}