Bash is a typical Unix tool, and Unix follows the KISS principle. Hence, also Bash scripts should be KISS.
If you need >100 lines of code your code is too complex in the script world, and you should split it into several scripts where each does one thing, and that one thing well. Usually, bash scripts are applied with pipes. A sequence like cat x | tr a b | sort >output is much more likely and easy to handle then a single script which does all these things.
KISS with Bash is a very different approach compared to other scripting languages like Python and Perl. There you can write long code easily and conveniently. However, things can get tough when larger scripts need to be maintained.
I consider the examples in the "Bash Bible" a collection of useful black boxes. It's fine if they just work. Regarding the "unreadable" trim_string for instance, if you have problems to understand that code, and you have to change something then you can simply write your own new trim_string script, even in Python or Perl if you like. Pipes work also well with them.
Update: Another advantage of bash and pipes over Python/Perl is that the Unix system can assign each script in a pipe to a separate thread. That means, simple bash scripts with pipes can work _much_ faster than single scripts in Python or Perl.