Ranked in these QuestionsQuestion Ranking
Pro Widely available
You can expect a reasonably recent version of Tcl to either installed or available in the repositories of any popular open source *nix.
Pro Standard library
Unlike *sh Tcl has a rich standard library.
With a little care you can have the same script work on Linux, *BSD, OS X and Windows.
Pro Tk and Expect
Pro Everything is a string
Tcl can operate at the same level of abstraction as the POSIX shell, which makes it easier to manipulate the output of other programs.
Pro Sane quoting rules
Unlike in *sh you don't need to quote your variable substitutions ('"$1"').