Simplicity is mIRC scripting's real strength though. It takes a few days of learning to be able to produce useful and meaningful results, the things which people point out as weaknesses (no OO, no data types etc.) are what allow that to happen. If you came to mIRC from a programming background then that simplicity may not help you, it may even make things worse, but the fact is that most people aren't coders - at least not when they first learn mIRC scripting.
What you call simplicity I call complexity. Please tell me how:
set %string_var %string_var $+ $chr(32) $+ some other text
is cleaner than:
$string_var .= " some other text"; (perl/php)
[append string_var " some other text"] (tcl)
string_var += " some other text" (python)
string_var << " some other text" (ruby)
How can you possibly tell me the mIRC syntax is clean and simple?
Documentation is another plus-point in mIRC's favour. Other languages might have more content relating to them on the web, but it's widely distributed and much of it is deprecated or poorly written. mIRC scripting is one of the few languages where documentation is instantly available in one single file on your computer from the get-go.
For perl help:
The perldoc program included with Perl provides extensive documentation on all aspects of the language.
Single file php help: http://www.php.net/distributions/manual/php_manual_en.chm
It documents ALL aspects of the php language, and unlike mIRC's helpfile, it tells you when a particular function was added to php so you know exactly what your target version must be.
Tcl help: http://prdownloads.sourceforge.net/tcl/tcl8.4.2-html.tar.gz
Python help: http://www.python.org/ftp/python/doc/2.2.2/html-2.2.2.zip
Ruby help: ftp://ftp.ruby-lang.org/pub/ruby/doc/ruby-man-1.4.6.tar.gz
I'm very familiar with the php helpfile and the tcl helpfile. I'm not really a python or ruby programmer so I can't comment on that too much. But the php and tcl helpfiles have answered any question I ever had. But in any case, there is something you neglected, books. Several php, perl, ruby, tcl, etc books have been published. There are no books on mIRC scripting. Books often help in ways online tutorials and documentation can't, but in the case of mIRC scripting this resource is unavailable; in the other languages, you have several dozens of books to choose from.