The \rewnewcommand\araystretch{1.1} is not cure for all cases ... your table is quit unusual, so it doesn't help in placing of text in multirow cells. In your case you need another aproach, for example use of gapedcells from package makecell:
\documentclass{scrartcl}
\usepackage{xltxtra}
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new]{german}
\usepackage{makecell}
\usepackage{multirow}
\usepackage{tabularx}
\usepackage{calc}
\begin{document}
\newcommand{\mytext}{And then it happened ... a door opened to a world ...
rushing through the phone line like heroin through an addict's veins, an
electronic pulse is sent out, a refuge from the day-to-day incompetencies
is a board is found.}
\setcellgapes{5pt}
\makegapedcells
\begin{tabularx}{\textwidth}{|X|X|X|X|X|}
\hline
\multirow{10}*{\parbox{\linewidth}{\mytext}}
&b&c&d&e\\\cline{2-5}
&\mytext
&c&d&e\\\hline
\end{tabularx}
\end{document}

Note: The \arraystretch work as name implied, in any array environment, from array, tabular to even tabu (most of them are based on array). Regarding columns width in your table: if it is not necessary that all to have the same width, you can change it, for example with >{\setlength\hsize{0.5\hsize}}X (to make it narrower) or even in case, that in columns are cells with only one line text use l column type. With this you can increase width of other X columns and obtain nicer text formatting in it.
Already now you can obtain slightly better text formatting, if you reduce font size and its shape:
{\sffamily\small
\begin{tabularx}{\textwidth}{|X|X|X|X|X|}
...
\end{tabularx}
}

Beside this is worth consider package ragged2e and its macro \RaggedRight. With it text is not justified anymore, but hyphenation still works. With it you obtain the following result:

(considered \sffamily\small fonts). Changes in above MWE are: added \usepackage{ragged2e} in preamble and in table:
\begin{tabularx}{\textwidth}{|X|X|X|X|X|}
\hline
\multirow{10}*{\parbox{\mrwidth}{\RaggedRight\mytext}}
&b&c&d&e\\\cline{2-5}
&\RaggedRight\mytext
&c&d&e\\\hline
\end{tabularx}
Addendum:
In all efforts to make good answers for you I forgot an very basic mutirow futures: defining it width with linewidth. This introduce p{\linewidth} type of cell, where the text is automatically broken in multi line text. So, the parbox seems to be not necessary (on this remind me Bernard answer, where this is used), however the hyphenation seems to be different. Difference is observable, if you futher decrease font size to footnotesize:

In above picture the first column is without use of \parbox. With its ˛use, the text shape in this column is the same as in the secon.
The use of \arraystretch doesn't work in my example because I don't put my tabularx inside a tabular or table environment.???\arraystretchworks withtabularxin the same way as it works with othertabularvariantsit does not need to be in a surroundingtableortabular` – David Carlisle Aug 28 '16 at 15:19