I can't speak for other scripting languages, but it's absurdly easy both to embed Tcl as an interpreter in a larger project, and to create a Tcl extension which implements your own commands.
You're using the same words. What is "easy" about embedding TCL that is "difficult" about embedding Perl or Python? Can you spell this out?
Sorry, missed this at the time. But I did say I can't speak for other scripting languages. All I can say is that it was much easier than I was expecting!
One can leverage Lua in PHP, I think it's handy for user-supplied logic (eg: business rules)