What is the "business case" for using Tcl?
There are lots of good things about Tcl. Very high quality engineering, a great C API, it's flexible, easy to use, widely deployed, and so on, although of course these are must be considered relative to other languages.
What are the features where Tcl really shines enough to make a case for it to those who don't really care what language is being used.
For instance, if a firm truly needs, say, a huge enterprise application server type system, Tcl probably isn't going to cut it for the core, because there exist other systems that are available to be deployed now. In Tcl you would have to build it.
On the other hand, where one needs a command language to embed in a product, it would be hard to beat Tcl.
What other areas do people feel Tcl is suited for in a context where a case must be made that it is going to be the best thing to use in terms of quality of results, and low cost?
LV Tcl was originally intended as an embeddable framework to build internal languages. That's one of the places where Tcl shines best. Perhaps others will add some of the many references to pages on this wiki that discuss the various positive reasons for using Tcl.