See:[#LPP-8151] instanceof on an undefined value will silently halt swf9 - Your Company JIRA http://bit.ly/l8Ydy
I think compiler.catcherrors is working well enough that it should be on by default in debug mode and that the flag compiler.catcherrors should be used if you want to leave that feature on all the time (in non-debug mode it makes DHTML and SWF9 behave more like SWF8, in that errors are silently ignored rather than halting the script execution).
