From an academic perspective I think it is totally worth the effort. It can teach you how to write a parser, a greater understanding of pseudo-TTYs, process IDs, and UNIX file streams (eg handling EOT bytes). Plus ANSI escape sequences (for readline support), globbing, etc. There's a lot of theory that can be learnt from writing even an unsuccessful shell. Even with myself who has been a Linux administrator for 10+ years and a developer for twice that, there were lessons I've learned when writing murex.