The best reasons to use Smalltalk are: 1) super clean syntax; 2) “live” coding and debugging; 3) OOP purity; 4) high productivity. Super clean syntax means that you can easily learn and master the language. (Try mastering C++ or Scala.) When you’ve mastered your tools, you can work at your maximum effectiveness and efficiency.
“Live” coding and debugging means that you have a very tight feedback loop for coding and testing. This means very fast and easy development.
OOP purity means that you can make the most of object orientation with late binding and across-the-board consistency. This results in great flexibility. Read Getting the Message.
High productivity means that you can easily meet deadlines and dramatically shorten “time to market.” Lower your development costs. Be very flexible with requirements. Read Smalltalk’s Proven Productivity.
Lisp also has a super clean syntax, but its prefix notation and torrent of parentheses make readability more difficult. Smalltalk reads more like pidgin English without the verbosity (it’s very concise). It feels more natural, and that’s why the syntax is highly prized.