Table column widths disproportionate due to multicolumn cell being too long

It's a feature of the \halign primitive:

\documentclass{article}
\usepackage{tabularx}
\begin{document}

\centering
\parskip2\baselineskip

\begin{tabular}{*5{c}}
\multicolumn{5}{c}{This is a piece of text. The longer it is, the longer the last column.} \\ 
1 & 2 & 3 & 4 & 5 \\
\hline
\end{tabular}


\begin{tabular}{*5{c}}
\multicolumn{5}{c}{\makebox[0pt]{This is a piece of text. The longer it is, the longer the last column.}} \\ 
1 & 2 & 3 & 4 & 5 \\
\hline
\end{tabular}


\begin{tabular*}{\textwidth}{@{\extracolsep{\textwidth minus\textwidth}}*5{c}@{}}
\multicolumn{5}{c}{This is a piece of text. The longer it is, the longer the last column.} \\ 
1 & 2 & 3 & 4 & 5 \\
\hline
\end{tabular*}


\begin{tabularx}{\textwidth}{*{5}{>{\centering\arraybackslash}X}}
\multicolumn{5}{c}{This is a piece of text. The longer it is, the longer the last column.} \\ 
1 & 2 & 3 & 4 & 5 \\
\hline
\end{tabularx}


\end{document}

Thank you so much David, this answer has helped me so much in using Excel2Latex. I now use this in my preamble

\usepackage{tabularx}
\usepackage{array}
\newcommand{\deftab}[2]{\begin{tabularx}{#1}{*{#2}{|>{\centering\arraybackslash}X}|}

and then use

\deftab{width}{no. of columns}

in place of the

\begin{tabular}

that Excel2Latex uses, and then I replace the necessary \end{tabularx} at the end of the table. It automatically fixes all of these types of problems in tables of any size and does so without disrupting any formatting.