I had to use TCL at a company where Prof. Ousterhout was on the board. I had used it a bit at university but I was coming from a Python job so, naturally, I hated its syntax BUT:

1. The project was in ancient C++ for many reasons and even the most modern C++ is at least 5x more difficult to write code in than TCL. So why write code that runs once in the life of the program in C++? There's often plenty of code that doesn't benefit from being compiled - that looks for a configuration file, reads and validates lines and calls functions to build initial datastructures...setup code, string handling. You save a millisecond or two by doing it in C++ but only once in a program that runs for hours - why waste all that dev/debug effort?

If you choose judiciously then you can end up doing a lot of your performance insensitive, setup-type code in TCL and a great deal of the time you'll be able to debug and fix bugs in an interpreted language without recompiling anything.

2. Python, though great, has a global interpreter lock. TCL fits in with threaded programs excellently. This can be a make-or-break feature.

3. TCL's syntax which I dislike, seems to map excellently into C - calling C from TCL just seems a lot easier than in python.