There were several other things the author mentioned--though I will admit I found that first part of the first reason somewhat silly.
The thing is, you really ought to expose yourself and at least play around in other languages. No language (except, perhaps, APL, assembly, Forth, Lisp, or Prolog [not that there exists a standard variant of the last two]) is perfect.
Every language does some things really well, other things really poorly, and muddles through the rest. Being familiar with other helps you know when your language of choice might need to call out to something better suited to your problem domain.
The thing is, you really ought to expose yourself and at least play around in other languages. No language (except, perhaps, APL, assembly, Forth, Lisp, or Prolog [not that there exists a standard variant of the last two]) is perfect.
Every language does some things really well, other things really poorly, and muddles through the rest. Being familiar with other helps you know when your language of choice might need to call out to something better suited to your problem domain.