I think the core ideas behind TCL are so simple and self-consistent that getting "deep" isn't required so much.

Instead it's just a case of using it. In the past I used TCL for "expect" and for the Tk graphical toolkit. I can't say I've written anything major in TCL, but I think that's a common-case. Most people use TCL because it is embedded in applications as a scripting language/DSL.

When people express interest in TCL I usually point them at antirez's simple writeup which gets a lot of the basics shown of in a simple manner:

http://antirez.com/articoli/tclmisunderstood.html

I think it really makes sense if you want an interactive shell. In contrast to most other languages, you need no parens or other fancy sigils for normal commands.

A classic use case is a debugger like gdb. What else, though?

I haven't written any Tcl in years but one of the things I liked about it was that it's not deep at all. Very simple rules, with enough flexibility built-in to (usually) make it do what you want.