To answer the "marketable skill" question... it depends. Tcl is used a lot in the QA world, and is also the industry standard scripting language in the EDA world. expect is the go-to tool for many sysadmins around the world (expect is a superset of Tcl). Also, Tcl is used as an embedded language in cisco routers, tivo, and other types of devices.
From a desktop or web application perspective, though, Tcl jobs are very few and far between. I worked steadily as a Tcl programmer for 15 years or so but eventually had to switch to Python out of economic necessity.
The best thing about learning Tcl, though, is that it opens your mind up to a different way to do things. The way Tcl works is fairly unique, and remarkably powerful and extensible. It's worth learning just to broaden your horizons a bit.