So what's the performance like, compared to the same Applescript implemented as an AppleScript script, a compiled AppleScript app, and as implemented in Ruby with Apple Event support?
Neither AppleScript nor Ruby are viewed as high-performance languages (admittedly, work is ongoing toward making Ruby faster). But the real reaason -- in the absence of performance concerns/constraints -- to select one scripting language over another is the ease with which it can be implemented and maintained.
While I routinely mix AppleScript with other scripting languages via the osascript facility in the shell environment and the AppleScript "do shell script" construct, I find that the best thing about AppleScript is that when coming back to a complex bit of code written with a lot of time between now and then, the AppleScript is always the easiest to understand what is going on in the code.
But I'm certainly not averse to extending the utility of Ruby in my scripting toolbox.