I don’t know why this feature is not implemented in LaTeX. You can very easily define your own commands, but they may take only a fixed number of arguments (up to 9). Bad luck, because the first command I needed to write for my dissertation had to have an optional argument:

Linguists traditionally write words and sounds from a language of interest in italics, e.g. the N|uu word ainki. Often it is followed by an enquoted translation, e.g. ainki ‘father’, xainki ‘mother’, n|ai ‘see’, and kx’ain ‘laugh’ all contain the diphthong ai.

So I wanted to write me a command \nuu by which I could write either just the N|uu word in italics or the word in italics followed by the enquoted translation. I would use it either just as \nuu{ainki} for ainki or as \nuu[father]{ainki} for ainki ‘father’.

Later, when I learn how to make an index of all N|uu words written in my document, I probably will just add an indexing command into the \nuu command definition. For now the optional argument was trouble enough. I did not expect that it will involve hardcore TeX programming.

Fortunately good people from the xetex@tug.org mailing list have suggested some solutions for this and the most bullet-proof and versatile was this (thanks to Morten Høgholm and Ross Moore):

\documentclass{article}

\makeatletter
\long\def\tlist@if@empty@nTF #1{%
\expandafter\ifx\expandafter\\\detokenize{#1}\\%
\expandafter\@firstoftwo
\else
\expandafter\@secondoftwo
\fi
}

\providecommand*\nuu[2][]{%
\textit{#2}%
\tlist@if@empty@nTF{#1}{}{ `#1'}% only the false code executed
}
\makeatother

\begin{document}
\nuu{ainki} \nuu[mother]{xainki} \nuu[goat]{mudi}
\end{document}

I use \providecommand instead of \newcommand. I feel safer. “It works like \newcommand, but if the command is already defined, LaTeX2e will silently ignore it.” (Oetiker et al., lshort.pdf, p. 109.)